Userspace programs can now open /dev/gui_events and read a stream of GUI_Event
structs one at a time.
I was stuck on a stupid problem where we'd reenter Scheduler::yield() due to
having one of the has_data_available_for_reading() implementations using locks.
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. :^)
