Fix invalid mouse press and release events on window maximize

Fix invalid mouse press and release events on window maximize

In Windows, if the window is maximized with a doubleclick on the title
bar or resized with a doubleclick on the bottom or top frame border,
there comes no WM_LBUTTONDOWN or WM_LBUTTONDBLCLK event for the second
click, but there comes at least one WM_MOUSEMOVE with left button down
and eventual WM_LBUTTONUP.
Since QWindowSystemInterface::handleMouseEvent() can't differentiate
between actual button down event and a move event with button held
down, the move resulted in a button down event being sent to the widget
that was under cursor after resize.

Fixed by ignoring any events with left button down for which we didn't
get the initial native button down event. Since entering an application
with mouse button already held down is not generating mouse events
anyway until the button is released, this should be safe.

Task-number: QTBUG-27845
Change-Id: Ibc7abf8a127c401801e1b654a2829582f43a1ae8
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>