Get rid of the goofy MouseClient interface and have the GUI event loop just
read mouse data from the character device.
The previous approach was awful as it was sending us into random GUI code
in the mouse interrupt handler.
Render the mouse cursor by xor'ing the pixels. I don't know anything about
hardware cursors yet and this way we don't need to recompose the window
hierarchy every time you move the mouse. :^)
