Use click shield for hit-tested evts

[#57656854]
swallow, don't hit test, clicks on click shield

[#57656854]

Doing simulated svg hit testing using "sandwiched" svg/canvas layers forces us to simulate clicks -- browser-issued click events will be completely spurious because the browser doesn't know the real target of the corresponding mousedown and mouseup events, and so can't determine whether the mouseup and mousedown were targeted at the same element, or not.

Note also that even if the above did not apply, the only sensible thing for the canvas to do with click events dispatched to it is ignore them anyway. Only the canvas based view can tell if the same atom received mousedown and mouseup.
Don't listen for touch events in canvas layer

[#57656854]

These are going to be handled at a higher layer, so the canvas layer will never see them.
Make canvas hit test call back during evt handling

[#57656854]
[#57817088]

This prevents mouse events which the canvas hit tests negatively and does not handle from bubbling to the window.
Dispatch click events ourselves when appropriate

[#57656854]

As previously mentioned, the browser-issued click event is useless to our svg/canvas/svg sandwich with simulated hit testing. But we can dispatch an informative click event ourselves. Notice that we do so in the same event turn as the mouseup; I suspect that's just fine in all cases.
Update setupHitTesting() comment to explain clicks

[#57656854]