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.

LibGUI/GEvent.h

@@ -131,7 +131,12 @@ private:
 
 class GTimerEvent final : public GEvent {
 public:
-    GTimerEvent() : GEvent(GEvent::Timer) { }
+    explicit GTimerEvent(int timer_id) : GEvent(GEvent::Timer), m_timer_id(timer_id) { }
     ~GTimerEvent() { }
+
+    int timer_id() const { return m_timer_id; }
+
+private:
+    int m_timer_id;
 };