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.
It was fun for everyone to share a single framebuffer but it was also
kinda really awful. Let's move towards having a "GraphicsBitmap" as the
backing store for each Window.
This is going to need a lot of refactoring so let's get started.