Android: Add system menu

The system menu opens with 3-finger touch. Currently, it can only exit
the game.

src/sys/mako.h

@@ -43,7 +43,7 @@ public:
 
 #ifdef _WIN32
 	void select_sound(BGMDevice dev);
-	void on_mci_notify(SDL_SysWMmsg* msg);
+	void on_mci_notify(const SDL_SysWMmsg* msg);
 #endif
 #ifdef __EMSCRIPTEN__
 	void select_synthesizer(bool use_fm);

src/sys/nact.h

@@ -267,7 +267,7 @@ public:
 
 private:
 	void pump_events();
-	void on_syswmevent(SDL_SysWMmsg* msg);
+	bool handle_platform_event(const SDL_Event& e);
 };
 
 class NACT_Sys1 final : public NACT {

src/sys/nact_input.cpp

@@ -19,6 +19,9 @@ void NACT::pump_events()
 {
 	SDL_Event e;
 	while (SDL_PollEvent(&e)) {
+		if (handle_platform_event(e))
+			continue;
+
 		switch (e.type) {
 		case SDL_QUIT:
 			terminate = true;
@@ -40,10 +43,6 @@ void NACT::pump_events()
 		case SDL_APP_DIDENTERFOREGROUND:
 			ags->flush_screen(false);
 			break;
-
-		case SDL_SYSWMEVENT:
-			on_syswmevent(e.syswm.msg);
-			break;
 		}
 	}
 }