Fix some coverity and cppcheck warnings (most are very minor issues: not initialized members and initialized but not used variables)

Fix some minor coverity warnings. Fix a bug in pcad2kicad plugin. Scripting: map BASE_SET. Rename some python bom scripts with better names. Fix erroneous comments and add comments

* Fix default KISYSMOD/KISYS3DMOD (#ifdef typo) for OSX builds.
* Remove setting KIGITHUB from kicad/pcbnew Info.plist in favor of hardcoded default
  setting for OSX builds.
* Remove now unneeded Set3DShapesDefaultPath() and other small things.

Fix some coverity warnings (not initialized members). Add comments. Better (more informative) title in dialog to download .pretty libs.

Move exact dialog: make all messages translatable. Fix minor issues (some can be Windows specific). Remove useless declarations. Fix coding style issues
Fix minor warnings from cppcheck.

Fix also very minor warnings detected by cppcheck.

* move not shared files (sch_item_struct.*, sch_base_frame.h) to eeschema;
* move wxEeschemaStruct.h to eeschema and rename it schframe.h to be consistent with the other corresponding file name schframe.cpp;
* remove few not needed  #include

Pcbnew: fix bug  #142348 (airwire does not always appear after deleting a track). Very minor other fixes.

Pcbnew: fix  Bug #1422093 (pcbnew crashes after editing a footprint cominng from the board  with the fp editor and saving it on a librray and trying to load it to the board)
Some other very minor fixes.

Now OpenGL can report its problems instead of shutting down the application.

Pcbnew: Fix Bug #1420074 (Crash when a footprint is added to the board (or edited) from the footprint editor, if a new pad was added, because the net info of the new pad is broken in this case)

Pcbnew: simplify draw code in  default canvas and plot functions by removing the plot LINE option, which is not existing in all plotters, not existing in opengl/cairo canvas, and not usefull in default canvas.

* Check to see if image handler is already initialized to prevent warning
  on debug builds.
* Change LIB_PART destructor message output to use wxLogDebug for debugging
  on windows builds.
* Minor code cleaning and coding policy fixes.

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