in most of files, including wx.h is not necessary, when only 2 or 3 wx files must be included.
Moreover, on windows, including wx.h sometimes create compil warnings about
shadowed vars defined in some specific windows headers.
panel_board_stackup.cpp: material selection:
replace wxChoice by a better widget (wxTextCtrl+wxButton) to call the dielectric manager dialog
Move dielectric material class to a specific file
In .gbrjob file, one can specify the thickness and color of some layers.
Currently, there is no way to enter these parameters.
This commit prepare a better support of .gbrjob files.
