Refactoring of GCode export of color changes, extruder switches etc,
so that the "color change" like extruder switches are applied first
at the Wipe Tower / G-code export, so that adding / removing
an extruder switch at the G-code preview slider does not invalidate
slicing.
1) Don't allocate ExtruderOverrides if not necessary
2) Use boost::container::small_vector<int32, 3) for ExtruderOverrides
(usually less than 4 instances are printed))
