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

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.

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.

* 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.

* With fixes from Adam Wolf.

Update ecc83 demo. default KISYS3DMOD definition on Linux: add /usr/local/kicad/share/kicad/module/package3d test to find a suitable value.

Page layout selection: fix Bug #1405972, and add minor enhancement in page layout selection dialog (the new layout is shown after selection).
When the filename is not absolute, the page layout file is now searched first in project folder, and then in kicad template folder, if not found in project.

Fix issue when using a page layout file in project folder: eeschema and Pcbnew did not use it, unless using an absolute path.
Now, if the file path is nor absolute, it is seen as relative to the project (which is the expected behavior)
fp lib wizard: when pcbnew id compiled with USE_GITHUB_PLUGIN=OFF, the github plugin option is no more accessible (and the web viewer no more used).

Allows use of modifier shift key with letters A ... Z  in hot keys (including hot key editor). Thanks to John Bird, who helped me with his patch.

Minor fixes. Remove unused icon. Add 3 new icons. Change one icon (3 icons are from Konstantin Baranowskiy)

Looked like a minor issue, but was due to a more serious bug, when using 2 different DSN_LEXERS which were not synchronized.
The fix is not perfect, but unfortunately, the parser used to read the PCB_PLOT_PARAMS in .kicad_pcb files is also used in legacy board file reader.
Therefore it is better than write 2 parser functions, one for legacy files, the other for the .kicad_pcb files, which make the same thing.

Grebview: add info in layer manager for Excellon drill filles, similar to X2 file function info in X2 gerber files.

Try to fix an issue on Ubuntu/Unity for some dialogs which are not displayed. For most, adding GetSizer()->Fit( this ) in dialog constructor fix the issue, but at least one dialog still is not displayed.

geometry: IsSolid() and Move() methods, segment overlap detection, some improvements in SHAPE_LINE_CHAIN class.

- fixed walkaround bug causing unwanted overlap/clearance violation when the first segment of trace being laid intersects the obstacle's hull at the same point twice (ie. goes in, turns around and goes out).
- fixed placer bug not splitting the start segment after toggling via placement or changing trace width

Fix many doxygen warnings (due to missing info, old comments, typo ...) when building the doxygen doc.

Fix minor issues about translated strings and coding style. Add comment relative to file function info  in X1 gerber file format.

Eeschema, Plot dialog: add Plot directory selection. The patch from 1383026@bugs.launchpad.net was widely used.

* Make all files generated by make_lexer() dependencies of thier respective build targets.
* Make building Boost a dependency for all libraries.
* Remove duplicate make_lexer() call for Specctra lexer files.