[UI] Implement keycode handling for Linux with additional keys from xenia#2234

src/xenia/ui/window_gtk.h

@@ -59,6 +59,10 @@ class GTKWindow : public Window {
   void BeginBatchedSizeUpdate();
   void EndBatchedSizeUpdate(WindowDestructionReceiver& destruction_receiver);
 
+
+  // Translates a gtk virtual key to xenia ui::VirtualKey
+  static VirtualKey TranslateVirtualKey(guint keyval);
+
   // Handling events related to the whole window.
   bool HandleMouse(GdkEvent* event,
                    WindowDestructionReceiver& destruction_receiver);