Every time I want to implement mouseMove: on a custom Morph, I find
myself needing to override #handleMouseMove:, because of some "rules"
mentioned in its comments mention:

	Rules say that by default a morph gets #mouseMove iff
		* the hand is not dragging anything,
		+ and some button is down,
		+ and the receiver is the current mouse focus.

Despite being a "system" level method, it looks like this one is
overridden all over the system more than any other Event handler and I
find myself wanting to do the same in three separate apps just so I
can get this event.  Does anyone know why the above conditions should
be at the "system" level instead of consistent with the other event
types?