WindowServer+LibGUI: Add a server-side clipboard.

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

WindowServer/WSClipboard.h

@@ -0,0 +1,29 @@
+#pragma once
+
+#include <AK/AKString.h>
+#include <WindowServer/WSMessageReceiver.h>
+#include <SharedBuffer.h>
+
+class WSClipboard final : public WSMessageReceiver {
+public:
+    static WSClipboard& the();
+    virtual ~WSClipboard() override;
+
+    bool has_data() const
+    {
+        return m_shared_buffer;
+    }
+
+    const byte* data() const;
+    int size() const;
+
+    void clear();
+    void set_data(Retained<SharedBuffer>&&, int contents_size);
+
+private:
+    WSClipboard();
+    virtual void on_message(WSMessage&) override;
+
+    RetainPtr<SharedBuffer> m_shared_buffer;
+    int m_contents_size { 0 };
+};