add (very) basic main menu

include/td/display/state/MainMenuState.h

@@ -1,15 +1,27 @@
 #pragma once
 
+#include <td/common/StateStack.h>
 #include <td/display/DisplayState.h>
 
 namespace td {
 
-class MainMenuState : public DisplayState {
+class MainMenuState;
+
+using MainMenuStateStack = StateStack<MainMenuState, void>;
+
+class MainMenuState : public DisplayState, public MainMenuStateStack {
   public:
+	using Menu = MainMenuStateStack::State;
+
 	MainMenuState(Display& a_Display);
 	~MainMenuState();
 
 	virtual void Update(float a_Delta) override;
+
+	void RenderBackButton();
+
+  protected:
+	virtual void OnKeyDown(SDL_Keycode a_Key) override;
 };
 
 }  // namespace td