Support for mouse buttons. The support is not complete, but it does

Support for mouse buttons. The support is not complete, but it does
more or less work. See x-mouse.scm for soem examples.

There is no support for chording (with multiple buttons, or with
button and key combos), multiple clicking, or dragging.

There is also no buffering of button events beyond the buffering done
by X itself. I'm not sure how much of a problem this is.

There is a small timing problem on edwin startup, where you can't
use a mouse button until a key has been pressed. I think that the
solution to this involves including mouse events (at least the button
events) in the input port abstraction.