JS puzzles: use the PointerEvent API if available.

If the browser knows what 'PointerEvent' means, then we switch our
'onmousefoo' event handlers to the 'onpointerfoo' events, for both the
puzzle canvas and the resize handle.

The immediate effect of this is that we get to use the
setPointerCapture method on the puzzle canvas, in preference to the
deprecated Firefox-only setCapture.

A pointer event also contains extra fields compared to a mouse event:
as well as telling you which pointing device the event comes from, it
can also provide extra information, such as pressure, or the angle of
a stylus if the hardware can detect it. I don't have any immediate
ideas about what those could be used for, but it can't hurt to have
them available just in case we think of something in future.

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