The pinch property and the pinch signal parameter are confusing,
especially since limits in the property don't apply to the values
in the signal parameter.

Task-number: QTBUG-37917
Change-Id: Id8fac60f505a1c54cc8debd3a29e013ca9d29773
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>
Reviewed-by: Martin Jones <martin.jones@jollamobile.com>