Prevent ending up in a state where focus is perpetually grabbed.

Prevent ending up in a state where focus is perpetually grabbed.

Mouse / enter / leave / key events etc are all blocked when a window has
the blockedByModalWindow flag set. The problem appears if a QWindow is
created and only later directly or indirectly parented to a modal window
that's currently showing. Since the decision on whether a window should
be blocked or not is based on its parent / transient parent chain, we
need to reevaluate the blocked status each time the parent or transient
parent of a window changes.

Task-number: QTBUG-26112
Change-Id: Ida6b118b556fe26d17fa86335a0fe7baddc7eaf8
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>
Reviewed-by: Bradley T. Hughes <bradley.hughes@nokia.com>