* Keep mixed-color filaments intact when the extruder count changes
The extruder-count spinner resized the filament arrays in bulk at the tail,
which is where mixed-color slots live, so a new filament landed behind the
mix and the sidebar skipped a slot number. It now adds and removes one slot
at a time through the same calls the sidebar's +/- buttons use, so a new
slot opens ahead of the mixed tail and a removal renumbers object filament
ids, painted facets, custom g-code and mixed components rather than
clamping them away.
Drops the vector overload of set_num_filaments(), which this leaves without
callers.
5552ed6cf1