WindowServer+LibGUI: Add a server-side clipboard.

On the client side, use GClipboard's data() and set_data(String) to access
the global clipboard. :^)

LibGUI/GClipboard.h

@@ -0,0 +1,14 @@
+#pragma once
+
+#include <AK/AKString.h>
+
+class GClipboard {
+public:
+    static GClipboard& the();
+
+    String data() const;
+    void set_data(const String&);
+
+private:
+    GClipboard();
+};