Add license browser, improve touchscreen UI

GpApp/MainMenuUI.h

@@ -0,0 +1,10 @@
+#pragma once
+
+struct Point;
+struct Window;
+
+void StartMainMenuUI();
+void TickMainMenuUI();
+void DismissMainMenuUI();
+void HandleMainMenuUIResolutionChange();
+bool HandleMainMenuUIClick(Window *window, const Point &pt);