FancyMainWindow: Polish dockwidget titlebar display

Pop up after 500 ms hovering over an active area of the
(approximate) size of the titlebar. Hide the titlebase
if the mouse leaves it. Completely hide it (as opposed
to the initial 3 px) if it's "inactive".

Change-Id: I9e43c7f501d2265466a7575da4e661e25aa6ccf5
Reviewed-by: Alessandro Portale <alessandro.portale@digia.com>