Fix tooltips without a transient parent

QToolTip has the same problems as QMenu, that is you can create one
without a parent and with a global position. Use the same trick and
use the window that last received an input event as the parent.

Change-Id: I093c8da0d54110903f35670b01dea6fa96abecf4
Reviewed-by: Robin Burchell <robin.burchell@viroteck.net>