Tighter parsing of PrusaSlicer's own G-code annotations
to avoid clashes with comments inside user G-codes.
Also the GCodeReader was extended to return string_views instead
of copying a substring, and the GCodeProcessor was partially adapted
to string_views.
1) All slicer's exceptions are now derived from Slic3r::Exception.
2) New exceptions are defined for slicing errors.
3) Exceptions are propagated to the Plater to show.
It remains to modify the slicing back-end to throw the new SlicingError
exceptions instead of std::runtime_error and to show the other exceptions
by a message dialog instead of a notification.
