Use format 4.6 (full resolution of Pcbnew) in Gerber files to avoid the creation of self-intersecting polygons.

Gerber output: Some changes after evaluation of Gerber files by Ucamco (the manager of GERBER format):
* default file ext is now .gbr (the "official extension")
* some outdated commands remoded.
* mainly use mm and 4.5 format instead of inches and 3.4 format. this is mandatory,
  because the 3.4 format (comming from old pcbned internal units) creates truncations in coordinates.
  (with serious issues in polygons which can appear self-intersecting afer truncation)

Export to Pcbnew is broken, but it is a very rarely used feature. It will be fixed later.

2) Fix passing wxString to common/config_params so lookup keys 
   can be dynamically generated.  This gives ~/.kicad files all the colors.
3) Fix LEGACY_PLUGIN so it loads all layer types and masks properly.
4) Extend the table in router/pns_index.h to 128 from 64.

2) Change from legacy Cu stack to counting down from top=(F_Cu or 0).
   The old Cu stack required knowing the count of Cu layers to make
   sense of the layer number when converting to many exported file types.
   The new Cu stack is more commonly used, although ours still gives
   B_Cu a fixed number.
3) Introduce class LSET and enum LAYER_ID.
4) Change *.kicad_pcb file format version to 4 from 3.
5) Change fixed names Inner1_Cu-Inner14_Cu to In1_Cu-In30_Cu and their
   meanings are typically flipped.
6) Moved the #define LAYER_N_* stuff into legacy_plugin.cpp where they
   can die a quiet death, and switch to enum LAYER_ID symbols throughout.
7) Removed the LEGACY_PLUGIN::Save() and FootprintSave() functions.
   You will need to convert to the format immediately, *.kicad_pcb and
   *.kicad_mod (=pretty) since legacy format was never going to know
   about 32 Cu layers and additional technical layers and the reversed Cu
   stack.

Minor enhancements.

They are now member of EDA_DRAW_FRAME.
This change could fix Bug #1330781 (Background color change in Eeschema affects background color in Pcbnew)
Show page limits has meaning only for the schematic editor,  the board editor and the page layout editor.
Draw background color selection exists only in eeschema and  page layout editor.
In pcbnew, only a black background is allowed.
A side effect is now the user should set the background color in schematic editor, and the library editor (2 different options).
The default is still the white color.

In KIFACE_I::end_common save the 'common' configuration variables loaded in KIFACE_I::begin_common (the background color and the page border flag)

- Refactored the common part of cursor key movement and crosshair update
  in the various GeneralControl
- Add x10 movement with the keyboard (CTRL modifier)
- Avoid fixup of the cursor position by dummy mouse movements generated
  by cursor warping (original analysis and idea Chris Gibson)
- Do key handling in a way to permit sub-pixel cursor movement 

   to a new BOARD in the footprint editor.  This is PAGE_INFO::A4 and company.
   
*) Remove static storage of the BOARD in the module editor.

Change class PROJECT to use a generalized wxString in place of the
RETAINED_PATH items, so that new session and project specific strings
of any purpose can be saved there, for the life of a session.

remove global g_UserLibDirBuffer which was a project specific global, and no longer used with FP_LIB_TABLE support.

Pcbnew: fix a minor bug: in zones with no net, only the zone clerance was used to created clearence,  regardless the pad or footprint local clerance.
Very minor other fixes.

Minor refactoring of PAINTER & RENDER_SETTINGS classes.

A couple of trivial accessors made inline

Pcbnew: plot dialog: remove obscure and useless option about texts which are not value or reference.
cosmetic enhancement in dialog.
page layout: fix bug Bug #1320839 (canvas refresh countinously called when moving an item)

Pull unfilled polygon drawing thru the line clipper; GTK mess up even in this case and the sheet border lines glitch on the screen at high zoom otherwise in pcbnew.

DXF export: fix incorrect export of polygons having thick outline (like zones): Thick segments of outline were drawn like lines with no thickness.
Fix ( workaround only) crash (Windows only) when a quasi modal frame (like footprint viewer) was called from a dialog (like the component properties dialog in schematic editor).
Very minor other fixes.