Implement event loop timers.

GObjects can now register a timer with the GEventLoop. This will eventually
cause GTimerEvents to be dispatched to the GObject.

This needed a few supporting changes in the kernel:

- The PIT now ticks 1000 times/sec.
- select() now supports an arbitrary timeout.
- gettimeofday() now returns something in the tv_usec field.

With these changes, the clock window in guitest2 finally ticks on its own.

Kernel/i8253.h

@@ -1,9 +1,12 @@
 #pragma once
 
-#define TICKS_PER_SECOND          600
+#include <AK/Types.h>
+
+#define TICKS_PER_SECOND          1000
 
 namespace PIT {
 
 void initialize();
+dword ticks_since_boot();
 
 }