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/GNotifier.h

@@ -0,0 +1,25 @@
+#pragma once
+
+#include <AK/Function.h>
+
+class GNotifier {
+public:
+    enum Event {
+        None        = 0,
+        Read        = 1,
+        Write       = 2,
+        Exceptional = 4,
+    };
+    GNotifier(int fd, unsigned event_mask);
+    ~GNotifier();
+
+    Function<void(GNotifier&)> on_ready_to_read;
+    Function<void(GNotifier&)> on_ready_to_write;
+
+    int fd() const { return m_fd; }
+    unsigned event_mask() const { return m_event_mask; }
+
+private:
+    int m_fd { -1 };
+    unsigned m_event_mask { 0 };
+};