DragHandler can affect Button's onClick event

See the attached `draghandlerbug.qml` file for example. In that example, there are three controls, the only difference between 1st one and 2nd one is the DragHandler.enabled, and difference between 1st and 3rd is the DragHandler's parent.

When clicking the colored rectangle, we expect getting "Button clicked" log from console, but the red one will get "TapHandler tapped" instead, other two are working as intended.