Pcbnew: fix minor bug about  plot line width.

* Fix a bug when full file name and path are passed to FOOTPRINT_INFO::
  ReadFootprintFiles() which I created in bug fix lp:593989.
* Fix a wxString debug assertion in EDA_APP::InitEDA_Appl() when the KICAD
  environment variable is defined as an empty string.
* Add error dialog when libraries cannot be found in system search path
  when loading footprint using the select footprint dialog.
* Add footprint library name column to the EDA_LIST_DIALOG when selecting
  footprints from the list.
* Allow reading all columns from the selected row in EDA_LIST_DIALOG.
* Remove redundant sort from EDA_LIST_DIALOG constructor
* Add library name member variable and accessors to FOOTPRINT_INFO.
* Make headers translatable for Eeschema select component from list dialog.
* Add some helper methods to FPID for identifying the FPID type and validity.
* Remove a bunch of trailing whitespace and add missing license comments.

Eechema: fix bug : when saving the schematic project, the lib cache was saved under the current sheet opened, not the root sheet.
Pcbnew: clamp default plot line width between 0.02 and 2 mm. the other bug (saving this parameter in internal units instead of mm is not fixed)

(explanation: float is implicitly promoted to double anyway, it's only useful
for storage; also a fp variable forces conversion of other int in the expression
without needing a cast)
Typo fixes in some comments 'floatting' -> 'floating' :D

Mass parameters in gerbers can't use scientific notation (altough some software 
allow it), so the %g format was not correct.

A specctra layer is not a LAYER_NUM, so that's probably why it was difficult to use.

The plan goes like this:
- eeschema still uses int in decidegrees
- all the other things internally use double in decidegrees (or radians
  in temporaries)
- in pcbnew UI the unit is *still* int in decidegrees

The idea is to have better precision everywhere while keeping the user with int i
angles. Hopefully, if a fractional angle doesn't come in from the outside, everything
should *look* like an integer angle (unless I forgot something and it broke)

When the time comes, simply updating the UI for allowing doubles from the user should
be enough to get arbitrary angles in pcbnew.

This is a starting point of some code enhancements relative to polygons in Pcbew and 3D viewer.

Adds "kicad_pcb" to the list of file-extensions for PCBnew in its Info.plist 
to make Launch Services recognize the file associations.

Change 8 bit string to UTF8 so fancy mew in "um" can be shown. 

Note: ArcTangente gives a double but *takes* ints, so it's not good to
use for nontrivial things

New conversion routines and sin/cos implementation for angles in decidegrees

* All items shapes are converted to polygons.
* Polygons are merged layer by layer (for calculation time reasons,zones are not merged)
* for copper layers, vias and pads holes are substracted from polygons (but, for calculation time reasons, not inside zones areas).
* the look is better, mainly when displaying the copper thickness
* solder and paste layers are now shown in 3D viewer.
* the code was seriously cleaned (but still needs to be enhanced).
* Note this is a work in progress which needs refinements.

* All items shapes  are converted to polygons.
* Polygons are merged layer by layer (for calculation time reasons,zones are not merged)
* for copper layers, vias and pads holes are substracted from polygons (but, for calculation time reasons,  not inside zones areas).
* the look is better, mainly when displaying the copper thickness
* solder and paste layers are now shown in 3D viewer.
* the code was seriously cleaned (but still needs to be enhanced).
* Note this is a work in progress which needs refinements.

- Removed spurious int casts (these are truncated anyway and will break
  doubles)

- Applied the Distance, GetLineLength, EuclideanNorm, DEG2RAD, RAD2DEG
  ArcTangente and NORMALIZE* functions where possible

- ArcTangente now returns double and handles the 0,0 case like atan2, so
  it's no longer necessary to check for it before calling

- Small functions in trigo moved as inline

Interior regions on Edge.Cuts are out as keepouts for specctra DSN file
and downstream routers.  Point to point matching during polygon assembly
can have individual thresholds, one for board edge and another for interior polygons.
This is more forgiving for sloppy graphical mousers, or those that work without
grid snap on.  Currently the threshold for board edge is at zero, meaning the
board edge has to be precisely defined as a polygon, just as before.  But it
is easily tunable for experimentation.

* Fixed a bug in PCB_EDIT_FRAME::loadFootprints when no footprint libraries
  are found when attempting to load footprints.
* Add a warning to PCB_EDIT_FRAME::loadFootprints to inform the user when
  a footprint library file cannot be found in any of the standard library
  search paths.
* Changed FOOTPRINT_INFO::m_padCount to unsigned to prevent signed/unsigned
  comparison compiler warnings.
* Put NestedSpace() function in netlist_reader.cpp inside conditional debug
  build statement to prevent warning in release builds.

  This seems to me like the defacto standard for this menu command.
* Added a default filename for save as. Currently it's set to
  "Unnamed file" using _() so its translatable.
* Changed the save dialog in PCBnew to use the path of the "original"
  file as a base for the new file.

* Fix both legacy and s-expression netlist readers when footprints are
  assigned in the netlist.
* Add some helper functions to NETLIST for detecting when footprints are set
  or not set and when they have been changed while loading the .cmp file.
* Rename a few functions to improve their readability.

In most case the assignment to null was not necessary since it was easily provable that the (local) variable wouldn't have referenced after that anyway.