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/GObject.cpp

@@ -60,12 +60,16 @@ void GObject::start_timer(int ms)
         dbgprintf("GObject{%p} already has a timer!\n", this);
         ASSERT_NOT_REACHED();
     }
+
+    m_timer_id = GEventLoop::main().register_timer(*this, ms, true);
 }
 
 void GObject::stop_timer()
 {
     if (!m_timer_id)
         return;
+    bool success = GEventLoop::main().unregister_timer(m_timer_id);
+    ASSERT(success);
     m_timer_id = 0;
 }