Handle pinch correctly when one point is released and repressed.

Handle pinch correctly when one point is released and repressed.

The pinch is not finished at the time one point is released.  Ensure
that a onPinchStarted is called each time a repress happens, and
that onPinchFinished is called when all points are released.

Fixes: QTBUG-19632

Change-Id: I467dd612383f7dd11d58a9df063dd86860796216
Reviewed-on: http://codereview.qt.nokia.com/2059
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Bea Lam <bea.lam@nokia.com>