Fix MouseArea stealing mouse grab after release.

This was preventing child items from receiving a release event and
making the MouseArea the mouseGrabber when the mouse wasn't depressed.

Change-Id: I93a9f4d6bd94041879ea13efda40c8d1dbb63a88
Reviewed-by: Mikko Harju <mikko.harju@jolla.com>
Reviewed-by: Martin Jones <martin.jones@qinetic.com.au>

src/quick/items/qquickmousearea.cpp

@@ -867,6 +867,7 @@ bool QQuickMouseArea::sendMouseEvent(QMouseEvent *event)
             break;
         case QEvent::MouseButtonRelease:
             mouseReleaseEvent(&mouseEvent);
+            stealThisEvent = d->stealMouse;
             break;
         default:
             break;