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.cpp

@@ -0,0 +1,15 @@
+#include <LibGUI/GNotifier.h>
+#include <LibGUI/GEventLoop.h>
+
+GNotifier::GNotifier(int fd, unsigned event_mask)
+    : m_fd(fd)
+    , m_event_mask(event_mask)
+{
+    GEventLoop::main().register_notifier(Badge<GNotifier>(), *this);
+}
+
+GNotifier::~GNotifier()
+{
+    GEventLoop::main().unregister_notifier(Badge<GNotifier>(), *this);
+}
+