xcb: mouse wheel does not focus a window

The window should react to the wheel event (e.g. scroll content)
but without becoming focused; this is the X11 convention.

Task-number: QTBUG-32517
Change-Id: I7e12425e5a6e1549b7f23dc318612a436c24d14b
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Check if the widgetItem is valid before accessing widget() on it

If a state was restored but not all the dockwidgets that were available
before are available at the time of restoration then when dragging a
dockwidget to where the unavailable one is expected to be would cause a
crash.

Change-Id: I829d93041b7950a3546ba4e6c3764b169f276315
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>