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Pcbnew: fix read .kicad_pcb file error when an outdated plot parameter is found. Now  outdated plot parameters are just ignored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

The default layer stack between these 2 types was swaped in pad definition, and also in the dialog pad properties.
This double swap creates no bug, but creates bugs in the footprint wizards (SMD or CONN pads do not have the right layer stack)
FYI, the CONN pad type should be removed soon.

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

Update complex_hierarchy.kicad_pcb file, used in qa tests, which was created by a test version of Pcbnew, and was not currently readable.

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* some outdated commands remoded.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)

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.  LAYER_NUM is only advisory and
   can safely be replaced with int.
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.

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