Start refactoring the windowing system to use an event loop.

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.

Kernel/Keyboard.cpp

@@ -114,7 +114,7 @@ Keyboard::~Keyboard()
 {
 }
 
-bool Keyboard::has_data_available_for_reading() const
+bool Keyboard::has_data_available_for_reading(Process&) const
 {
     return !m_queue.is_empty();
 }