bugfix: changing track width/via size using the dropdown menu when custom size was enabled did not change anything.

Reduced number of switched events, allowing to use VIEW_CONTROLS, even if there is no extra event dispatcher.

Event handlers are (dis)connected depending on the active view.
TOOL_DISPATCHER inherits from wxEvtHandler, so now it receives events directly instead of being fed by external handlers.

Minor changes (removed an unnecessary event handler and two function calls that had no real influence).

Moved View related routines & fields from PCB_{BASE,EDIT}_FRAME & BOARD to PCB_DRAW_PANEL_GAL.

Removed EDA_DRAW_FRAME::SetGalCanvasActive() [it may be misleading], added EDA_DRAW_PANEL_GAL::GetBackend().

*) re-enable the MacOpen() support.
*) fix path truncation bug.
*) open *.kicad_pcb and *.sch files in the same process if they are part of the 
   the currently open project, even from the tree view.

switch back to original sexpr usage of PTREE, add new DSNLEXER constructor, enforce accessors in kicad.exe

Pcbnew: fix read .kicad_pcb file error when an outdated plot parameter is found. Now  outdated plot parameters are just ignored

Pcbnew: Rework on Gerber ouput: allows choice between format 4.5 and 4.6. Fix a minor issue in Gerber layers attributes.
fix print issue when printing each layer on a separate page.

  panel
- Corrected handling of scrollability in layer panel (fixes 1336996)

Also a minor cosmetic enhancement in SVG export and plot dialogs.