expose mouse button and modifiers from underlying AWT mouse event

[dandrake]

Fixes #28.

We now have getButton and getModifiers methods and some associated constants. I also added in getEventId, though that looks to InputEvent.getModifiers which was deprecated a very, very long time ago; we should consider changing that to use getModifiersEx.

[dandrake]

I tested this and it works as expected. I am looking for feedback on several bits, though:

• I copied over a bunch of the constants from MouseEvent; on the other hand, users can access those with MouseEvent.BLAH and similar. But maybe that's too confusing -- they won't have a good notion of the difference between Kilt's MouseButtonEvent and the AWT MouseEvent.

• I also expose the modifiers, but those are bitmasks and correctly interpreting them using & and | operators will be tricky for students. OTOH I don't think many of them will ever need or want to do that kind of handling.

• I also expose the event ID, but I'm not sure if that'll be useful.

[dandrake]

For the failing regression tests -- on my Mac, I got that error and fixed it by changing the required Java version from 17 to 21. Looks like the failing tests did so because they do have version 17 -- do we need to still support that? I suspect so, but...maybe not?

[dandrake]

Per Paul's comment, it seems our approach is to suppress as much of the underlying AWT stuff as possible. So it seems fine to copy over all the event, button, and mask constants.