in childMouseEventFilter, touchpoint grabber is also the mouse grabber

When an item (such as Flickable) filters its children's mouse events,
sometimes the mouse event may be one that is synthesized from a touch
point.  If that touch point is already grabbed, then in the context
of childMouseEventFilter QQuickWindow::mouseGrabberItem() should return
the item which has grabbed the touchpoint from which the mouse event
was synthesized.  Otherwise, there was a regression in which an item
which can be dragged via touch (such as Slider in QQ Controls 2)
could have its grab stolen by a filtering parent, such as Flickable,
or the gesture recognizer in QtLocation.  mouseGrabberItem() was
returning null because touchMouseId and touchMouseDevice were not
set, and the actual mouse was not grabbed.

If a touch event is used to synthesize a mouse event, and during
delivery an Item steals the grab of the synthetic mouse event, the
original grabber needs to be notified that it has lost the grab.

Task-number: QTBUG-59416
Change-Id: Ib121b06121df7593c0d549a6df42397b8ead1c45
Reviewed-by: Paolo Angelelli <paolo.angelelli@qt.io>

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