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.cpp

@@ -0,0 +1,48 @@
+#include <WindowServer/WSClipboard.h>
+
+WSClipboard& WSClipboard::the()
+{
+    static WSClipboard* s_the;
+    if (!s_the)
+        s_the = new WSClipboard;
+    return *s_the;
+}
+
+WSClipboard::WSClipboard()
+{
+}
+
+WSClipboard::~WSClipboard()
+{
+}
+
+void WSClipboard::on_message(WSMessage&)
+{
+}
+
+const byte* WSClipboard::data() const
+{
+    if (!m_shared_buffer)
+        return nullptr;
+    return (const byte*)m_shared_buffer->data();
+}
+
+int WSClipboard::size() const
+{
+    if (!m_shared_buffer)
+        return 0;
+    return m_contents_size;
+}
+
+void WSClipboard::clear()
+{
+    m_shared_buffer = nullptr;
+    m_contents_size = 0;
+}
+
+void WSClipboard::set_data(Retained<SharedBuffer>&& data, int contents_size)
+{
+    dbgprintf("WSClipboard::set_data <- %p (%u bytes)\n", data->data(), contents_size);
+    m_shared_buffer = move(data);
+    m_contents_size = contents_size;
+}