Provide a way to cancel on-going gestures in a context

Currently, there is no way to cancel gestures in a context from the application code. It is possible to set gesture cancel policy, but the policy triggers only when gesture is in started state.

Without a possibility to cancel, some use-cases become hard/impossible to implement, for example:

• a generic container widget, that changes to edit mode when long-pressing.

• To signal user that long-pressing does something, the gesture needs to be started early (container widget shows visualization when it receives Qt::GestureStarted)
• However, long-press might get canceled
• gestures related to the widgets within the container need to be canceled when entering edit mode.

• Canceling on-going gestures when a context menu popup is shown

Possibly related tasks:
http://bugreports.qt.nokia.com/browse/QTBUG-8261
http://bugreports.qt.nokia.com/browse/QTBUG-8177