Doc: Clean up SensorGesture QML example

Fix warnings, fix property names and strings shown in
the UI. Also touch the documentation and correct a
typo in the qdoc file name.

Task-number: QTBUG-33877
Change-Id: Ie790708ac4d1f8d72af03b52ee361d0bf3994adc
Reviewed-by: Lorn Potter <lorn.potter@jollamobile.com>