Port Terminal to LibGUI.

To facilitate listening for action on arbitrary file descriptors,
I've added a GNotifier class. It's quite simple but very useful:

GNotifier notifier(fd, GNotifier::Read);
notifier.on_ready_to_read = [this] (GNotifier& fd) {
    // read from fd or whatever else you like :^)
};

The callback will get invoked by GEventLoop when select() says we
have something to read on the fd.

LibGUI/GEventLoop.h

@@ -1,11 +1,13 @@
 #pragma once
 
 #include "GEvent.h"
+#include <AK/Badge.h>
 #include <AK/HashMap.h>
 #include <AK/OwnPtr.h>
 #include <AK/Vector.h>
 
 class GObject;
+class GNotifier;
 class GWindow;
 struct GUI_Event;
 
@@ -27,6 +29,9 @@ public:
     int register_timer(GObject&, int milliseconds, bool should_reload);
     bool unregister_timer(int timer_id);
 
+    void register_notifier(Badge<GNotifier>, GNotifier&);
+    void unregister_notifier(Badge<GNotifier>, GNotifier&);
+
     void exit(int);
 
 private:
@@ -64,4 +69,5 @@ private:
     };
 
     HashMap<int, OwnPtr<EventLoopTimer>> m_timers;
+    HashTable<GNotifier*> m_notifiers;
 };