LibCore: Move LibGUI/GNotifier to LibCore/CNotifier.

LibCore/CNotifier.h

@@ -0,0 +1,26 @@
+#pragma once
+
+#include <AK/Function.h>
+
+class CNotifier {
+public:
+    enum Event {
+        None        = 0,
+        Read        = 1,
+        Write       = 2,
+        Exceptional = 4,
+    };
+    CNotifier(int fd, unsigned event_mask);
+    ~CNotifier();
+
+    Function<void()> on_ready_to_read;
+    Function<void()> on_ready_to_write;
+
+    int fd() const { return m_fd; }
+    unsigned event_mask() const { return m_event_mask; }
+    void set_event_mask(unsigned event_mask) { m_event_mask = event_mask; }
+
+private:
+    int m_fd { -1 };
+    unsigned m_event_mask { 0 };
+};