Implemented the LAYER_NUM typedef (LAYER was already taken as a class name...) to represent a layer number.

(meaning was inverted)

- Renamed ReturnMaskLayer to GetLayerMask (since it's a plain getter)

pcbnew: fix some cosmetic issues mainly on Linux ( these issues could be very dependant on wxWidgets/Gtk versions)

Fix many issues in config functions, mainly a rounding issue when a double is stored in wxConfig file (was stored only using 4 digits in mantissa)
Remove dead or obsolete code.

Fix rounding issue when a double is stored in a wxConfig file (wxWidgets 2.9.4 store only 4 digits in mantissa).
A new inline function ConfigBaseWriteDouble( config, key,  double_value )  is used instead of config->Write(  key, double_value ) which store 12 digits

pcbnew: fix print format in THROW_IO_ERROR
        fix properties file export_vrml.cpp

Drc_On => g_Drc_On
Route_Layer_TOP => g_Route_Layer_TOP
Route_Layer_BOTTOM => g_Route_Layer_BOTTOM
Track_45_Only_Allowed => g_Track_45_Only_Allowed

Pcbnew: fix an issue when texts in footprints have a size = 60 mils (default size for texts). After saving and reading the .kicad_pcb file, the size was not correct.

Pcbnew: Fix a collision name between 2 methods: EDA_ITEM::GetFlags() and ZONE_CONTAINER::GetFlags(), which creates a very strange behavior when creating/edition zones.
ZONE_CONTAINER::GetFlags() is now named  ZONE_CONTAINER::GetLocalFlags().

* Complete encapsulation of the ZONE_CONTAINER class.
* Fixed a bug in the EDA_TEXT::Format() default effects behavior due to
  improper conversion to internal units and missing test for default
  thickness.
* Improved grammar of some Pcbnew context menu strings.
* Moved some more cleverly hidden BOARD object methods into class_board.cpp.
* Lots of coding policy and documentation fixes.