Fixed return type of std::string translate_utf8() with context
(incorrectly returned wxString, should return std::string).
Fixed double translation of BackgroundSlicingProcess::validate() in Plater.
Introduced a new localization macro L_CONTEXT(string, context)
to define context dependent translations.
The only strings - "Top" and "Bottom" were already using context, but
until now the context had to be added to the PrusaSlicer.pot file
manually, while the context is being generated automatically now.
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}
YuSanka