[Platform] Don't show drag-drop dialog unless needed

Don't show drag-drop dialog (move/copy/link)
unless there is actually a choice for the
user (that is, only show it one more than
one of those actions is allowable.)
[Platform] Test drag-drop handler

Test drag-drop handler. Initial test simply
ensures that three options are presented.
[Platform] Add drag-drop handler tests

Add test cases for the drag-drop handler
to satisfy code coverage standards.