Changed state in MouseRelease so that QTreeView
does not end up in wrong state if drag is started
but not completed.

Task-number: QTBUG-44773
Change-Id: I0b665d2944f2b696bc4b7e79689d110aefa3f194
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>
Reviewed-by: Thorbjørn Lund Martsum <tmartsum@gmail.com>