Fixed crash on destruction of animating QDockWidget in a QMainWindow

Fixed crash on destruction of animating QDockWidget in a QMainWindow

It doesn't make sense to hold an unguarded pointer to a
QPropertyAnimation while assigning ownership of that animation to the
animated widget.

Destruction of the widget while the animation is in progress causes
the animation pointer to become dangling; then the widget is removed
from the containing QMainWindowLayout, which attempts to abort the
animation, dereferencing the invalid pointer.

The crash can be reproduced sometimes with
tst_QDockWidget::taskQTBUG_2940_resizeAfterUndocking (which is in
Qt4 only).

Change-Id: I758bf7193b2ea39cd4d8e87197d8ff957d3368eb
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>
Reviewed-by: Andy Shaw <andy.shaw@digia.com>