allow DragHandler to be used as an attached property

This appears to be one of the trickiest tasks we have left in Pointer Handlers. TapHandler is exposed as the Tap attached property; PinchHandler is exposed as the Pinch attached property (at least, I've had unapproved patches for those sitting around for months). But MouseArea already has a Drag attached property, for Drag and Drop support, so we can't easily use the same name for an attached DragHandler.