Children widgets of drag handle also handle mouse down event.

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Just like in this demo https://gwt-dnd.appspot.com/#DragHandleExample i have a 
window with a dra handle. But this drag handle also has child widgets: buttons, 
edit box, etc. 

When i press mouse button down on one of those child widgets, the whole 
draggable panel gets dragged.

How do i prevent dragController.makeDraggable(panel, handle) not to register 
onMouseDown event for the child widgets of handle widget?

Original issue reported on code.google.com by balciuna...@gmail.com on 21 Feb 2014 at 7:19