LibGUI: Add Clipboard::clear()

Does exactly what it says on the tin :^)

Userland/Libraries/LibGUI/Clipboard.cpp

@@ -78,6 +78,11 @@ void Clipboard::set_data(ReadonlyBytes data, const String& type, const HashMap<S
     connection().async_set_clipboard_data(move(buffer), type, metadata);
 }
 
+void Clipboard::clear()
+{
+    connection().async_set_clipboard_data({}, {}, {});
+}
+
 void ClipboardServerConnection::clipboard_data_changed(String const& mime_type)
 {
     auto& clipboard = Clipboard::the();

Userland/Libraries/LibGUI/Clipboard.h

@@ -22,6 +22,7 @@ public:
     ByteBuffer data() const { return data_and_type().data; }
     String mime_type() const { return data_and_type().mime_type; }
     void set_data(ReadonlyBytes, const String& mime_type = "text/plain", const HashMap<String, String>& metadata = {});
+    void clear();
 
     void set_plain_text(const String& text)
     {