Reworked skirt generator to only generate skirt at non-empty layers
(layers that actually extruder object or support) and to respect minimum
layer height. Skirt generator stops at the first layer where those both
conditions cannot be met.
psWipeTower now equals to new psToolOrdering indicating that
the ToolOrdering has been calculated (only if non-sequential mode is active).
psBrim now equals to new psExtrusionPaths
psExtrusionPaths shall be the last step before psWipeTower, indicating
that all the printing extrusions are calculated for the G-code preview
slider to edit the custom per print_z color changes, tool changes etc.
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))
