Reworked the way of processing events in TOOL_MANAGER class.
Added GetCommandStr() for TOOL_EVENT class.

Introduced a new type of action: TA_ACTIVATE to distinguish events activating tools from common tool actions.

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().

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

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.

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* some outdated commands remoded.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)

Pspice netlist generator: add option to use net numbers as net names (feature removed a long time ago, but needed by Oscad team)
Note these options are saved in project config.