Pcbnew: fix Bug #1413701 (Segfault on highlight "unconnected" net 0 in Pcbnew ). Also, remove now useless code and other very minor fixes.

Pcbnew: fix issues and potential issues when reading board files and mainly old board files, and enhance the DRC tests and Delete Single Pad Net option when reading a netlist:
* Delete Single Pad Net option does not delete the net if a zone use this net (i.e. is attached to this pad).
* pcb_parser accept now negative netcodes in zones (can happen with old files, which previously could crash Pcbnew)
* pcb_parser accept now files with incorrect or missing net count  (can happen with old files, which previously could crash Pcbnew)
* if a zone has a non-existent net name it now keep this net name, and DRC detect it (previously, the net name was lost, and the DRC did not tected this issue).
* Drc test: now detect a "dead" net, i.e. a net with 0 pads, but still used by a zone. It happens easily after a schematic modification, when a net disappears or is renamed.

Eeschema: fix a minor bug and enhances SCH_MARHER::Matches function (  partial use of  0005-SCH_MARKER-fix-a-Woverloaded-virtual.patch, from <fatzer2@gmail.com> ) and other very minor fix.

Pcbnew, layers manager: Fix an annoying bug I added in rev 5359, in Render options: in Board editor many render options were not displayed (similar fp editor mode).
Now fixed.

Pcbnew: Fix potential bug in DRAG_SEGM_PICKER (a variable type bool was used as int. type is now int). Minor code cleanup.

Very minor fixes: add missing const return type to some GetBoundingBox() declarations. Change few functions names to better names in some places (like GetName() to GetPinName() )

Minor and very minors fixes: move -DKICAD_USE_WEBKIT definition to the main CMakeLists.txt. Remove duplicate semi-colon at end of some lines.

* if the current select plugin is the github plugin, one can select some of these libraries and add them to the table
* if the current select plugin is the kicad plugin, one can select some of these libraries and download them to make alocal copy.
  They can added to the table after they are downloaded.

Fixes issues with IDF file I/O on Windows (the famous 'ascii vs. binary' issue) and the reported problem of partial paths when KISYS3DMOD is specified.

Cvpcb: fix a typo which prevents cvpcb to be compiled with webkit suppport. Equ files support enhancements.

* Make generate button the default in Eeschema netlist dialog.
* Fix spacing issues in export IDF dialog.
* Make OK button the default in export IDF dialog.
* Remove unnecessary end dialog code from export IDF dialog.
* Improve resizing of export IDF dialog.

* Add dialog to edit the list of active equ files.
* allows use of our environment variables in .equ file names.

Add hight contrast option in Modedit. In hight contrast mode, solder masks and solder paste are shown.
this option was possible only by modifying the way display options are managed.
before these changes, display options were a global DISPLAY_OPTIONS class instance.
Now each EDA_DRAW_FRAME(and derivated classes)  includes its own DISPLAY_OPTIONS class instance.
As a consequence, some duplicate display option variables in these classes have been removed, because there were just duplicate variables of the  DISPLAY_OPTIONS class instance.

Modedit: now shows a specific layers manager, which allows user to select the technical layer which is used when adding a new graphic text or outline to the current footprint.
Please, note the layer manager was already available in GAL mode, but is was the same as tne board editor, which is not good.
Now:
- In default mode or GAL mode, the look is the same.
- Only suitable technical layers (i.e. paired layers) are selectable from the layers manager (for special purposes, some other layers can be used, via the graphic items properties dialog)

* With fixes from Adam Wolf.

Pcbnew: better display of tracks/vias/grid in toolbar. All: modify the zoom value shown in toolbar or status bar (roughly now zoom 1.0 is near the 1:1 scale, on a 22 inches Monitor). Fix also many other minor and very minor issues

* Change Read to Load in Pcbnew and Eeschema preference menus.
* Make electrical rules check status bar message the same for both the menu
  and toolbar entries.