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/GEventLoop.h

@@ -1,6 +1,7 @@
 #pragma once
 
 #include "GEvent.h"
+#include <AK/HashMap.h>
 #include <AK/OwnPtr.h>
 #include <AK/Vector.h>
 
@@ -23,6 +24,9 @@ public:
 
     bool running() const { return m_running; }
 
+    int register_timer(GObject&, int milliseconds, bool should_reload);
+    bool unregister_timer(int timer_id);
+
 private:
     void wait_for_event();
     void handle_paint_event(const GUI_Event&, GWindow&);
@@ -30,6 +34,8 @@ private:
     void handle_key_event(const GUI_Event&, GWindow&);
     void handle_window_activation_event(const GUI_Event&, GWindow&);
 
+    void get_next_timer_expiration(timeval&);
+
     struct QueuedEvent {
         GObject* receiver { nullptr };
         OwnPtr<GEvent> event;
@@ -38,4 +44,19 @@ private:
 
     int m_event_fd { -1 };
     bool m_running { false };
+
+    int m_next_timer_id { 1 };
+
+    struct EventLoopTimer {
+        int timer_id { 0 };
+        int interval { 0 };
+        timeval fire_time;
+        bool should_reload { false };
+        GObject* owner { nullptr };
+
+        void reload();
+        bool has_expired() const;
+    };
+
+    HashMap<int, OwnPtr<EventLoopTimer>> m_timers;
 };