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In Firefox, Blockly seems to handle mouse events differently than in
Chrome. This generally isn't a problem, but it affects the warning
indicator toggle button. Blockly starts a drag at the mousedown but
then we never will see a mouseup or click event.

This change registers a mouse down listener on the toggle to stop
propagation to Blockly so that the warning indicator can receive the
click event. We also update to include support for touch events on the
toggle button to support App Inventor on tablets.

Change-Id: Iba4e1e274dd3eac86939bb122a1c7f15cc7d41f9