If an item is clicked to trigger it, clear the rest of the selection

Note that the clearing is not done on mouse press, but on mouse release.
The reason is that there are situations when multiple items are selected
and pressing the mouse on one of them should not clear the selection
(e.g., drag and drop of multiple items).


REVIEW: 103703
FIXED-IN: 4.8.0
(cherry picked from commit 9f711b5f2e1d1fd856cd6b033e6adb96f9b46d8a)