Added synchronization of GCodePreviewData between the front end / back end
(GCodePreview data is only used if PrintStep psGCodeExport is finished).
Added reset of GCodePreviewData on Print::apply() to conserve RAM.
Removed libslic3r from the default include paths for all modules but
libslic3r. Now headers from libslic3r need to be included with an
explicit path (libslic3r/libslic3r.h etc)
Split the localization macros into I18N.{cpp,h}
replaced Print & SLAPrint pointers with BackgroundProcessing pointer
at the GLCanvas3D level, so that the Print & SLAPrint access will be
synchronized with the current printer technology.
