and for separation of concerns:
The final G-code preview no more uses ExtrusionPaths structure
to hold the G-code path data extracted by parsing the G-code.
Instead, the ExtrusionPath class has been trimmed down back to
the original size before the G-code preview was introduced,
and a new GCodePreviewData::Extrusion::Path class was created to hold
the additional path data as the extruder ID, color change ID
and fan speed.
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}
