Handle MouseArea.enabled = false after mouse is pressed.

Currently this leaves the MouseArea in a broken state,
i.e. still in pressed state, and the next press after
it is re-enabled is ignored.

In this case we now allow subsequent mouse move or
release events to continue. Following the release,
no further press will be accepted.

Change-Id: I65a890da90e2166ad568505fffdbd3db6c97165b
Reviewed-by: Andrew den Exter <andrew.den-exter@nokia.com>