Make mouse operations on grouped regions apply to pre-existing selection

On clicking on a region that is part of an active edit group with the
'selection' property enabled, its equivalent regions also become selected.
Thus, there's no need to reset the selection to the equivalent
regions when performing mouse drags on the region, and trim/fade in/fade
out/move/copy will always operate on the existing region selection if there
is one.

Fixes #5515.