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.

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

Use format 4.6 (full resolution of Pcbnew) in Gerber files to avoid the creation of self-intersecting polygons.

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* some outdated commands remoded.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)

Export to Pcbnew is broken, but it is a very rarely used feature. It will be fixed later.

2) Fix passing wxString to common/config_params so lookup keys 
   can be dynamically generated.  This gives ~/.kicad files all the colors.
3) Fix LEGACY_PLUGIN so it loads all layer types and masks properly.
4) Extend the table in router/pns_index.h to 128 from 64.

2) Change from legacy Cu stack to counting down from top=(F_Cu or 0).
   The old Cu stack required knowing the count of Cu layers to make
   sense of the layer number when converting to many exported file types.
   The new Cu stack is more commonly used, although ours still gives
   B_Cu a fixed number.
3) Introduce class LSET and enum LAYER_ID.
4) Change *.kicad_pcb file format version to 4 from 3.
5) Change fixed names Inner1_Cu-Inner14_Cu to In1_Cu-In30_Cu and their
   meanings are typically flipped.
6) Moved the #define LAYER_N_* stuff into legacy_plugin.cpp where they
   can die a quiet death, and switch to enum LAYER_ID symbols throughout.
7) Removed the LEGACY_PLUGIN::Save() and FootprintSave() functions.
   You will need to convert to the format immediately, *.kicad_pcb and
   *.kicad_mod (=pretty) since legacy format was never going to know
   about 32 Cu layers and additional technical layers and the reversed Cu
   stack.

Minor enhancements.