Support optionally flushing down:
- faces -> edges & verts.
- edges -> verts.
For C++ selection logic flushing down is avoided as it's an extra step
and instead, selection logic must take care to de-select edges/faces
before selection, so any shared vertices or edges are selected.
Flushing down means scripts can set the selection on faces or edges,
then flush this to all other elements without being concerned with
the order they have been set.
Ref !147351
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