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.

VirtualFileSystem/NullDevice.cpp

@@ -12,7 +12,7 @@ NullDevice::~NullDevice()
 {
 }
 
-bool NullDevice::has_data_available_for_reading() const
+bool NullDevice::has_data_available_for_reading(Process&) const
 {
     return true;
 }