Use selection->regions instead of Editor::get_regions_from_selection()

Remove Editor::get_regions_from_selection(), and use selection->regions
directly in the few places it was called.
Ignore track selection if there are any selected regions.

Make Editor::get_regions_from_selection_and_edit_point() only consider the
selected tracks when finding regions to operate on if there are no regions
selected, and never if the edit point is 'mouse'.
Don't include unselected regions in edit grouped region operations.

Make Editor::get_regions_from_selection_and_entered() only return regions
that are actually selected or entered, not their unselected equivalents.