Use Alt-LMB for selecting in vertex/weight paint modes for vertex, face
& pose bone selection - replacing Ctrl which was previously used for
selecting. This is needed so Ctrl & Shift can be consistent between all
paint modes.
Loop selection is available With Alt-LMB but only when selection tools
are active (with the LMB select key-map). They're always available with
the RMB select key-map.
This also resolves a conflict where box & lasso select tools couldn't
be used to de-select (via Ctrl) because the Ctrl-LMB was also clearing
the selection.
Part of #105298 design task.
fb54d3f865