`SC.RootResponder` mistook two successive mousedown events at the same point as a double click even if the target of the second mousedown was different. The result was that if a view moved suddenly on a first click, a second immediate click at the same point would try to call `doubleClick` on the new target view instead of just `mouseUp`.
Tidies up some collection view mouse handling tests.
Tidies SC.CollectionView a little bit.
Improves the double click rejection handling from previous change in SC.RootResponder. It checks the SC.View handler rather than the evt.target, because some views like SC.ListItemView replace their layer on update, so a double click would fail even though it's the same target view instance.
Fixed a regression in SC.CollectionView that prevented double clicks from triggering the action. Closes #1304 & #1305.