Fix mouse event distribution for Flickable with pressDelay

Fix mouse event distribution for Flickable with pressDelay

If an item responds to mouse events but does not accept them, it
can prevent the events from being processed by the correct item
further up the parent chain.  For example, a text item inside a
mouse area can wrongly consume a press event, so that the following
release event does not yield a click when processed by the mouse area.

Rather than speculatively assigning the mouse grab to items during
event filter processing, change Flickable to retain the grab for
the duration of the pressDelay and to release it during replay of
the press event.

Change-Id: Ied12b9643838a984c7026978047465c2830e55e4
Reviewed-by: Martin Jones <martin.jones@jollamobile.com>