Mousemove events do not have the correct button information
Need to keep the state of which button is clicked at which moment
This was actually the initial implementation ...
But I removed it because I couldn't remember why we did this in the
beginning.
Added lots of 'warnings' in comments, but won't be safe until we get good
integration tests.
References :
- FF : https://bugzilla.mozilla.org/show_bug.cgi?id=297919
"What if multiple buttons are pressed during mouse move? And how does the
integer field (button) indicate that no button is pressed? I think this
should be wontfix -- if people need button information during mouse move events
we need a new way of getting it (buttonSet), not the same way it works for click,
etc."
- IE : couldn't find any reference ...
