Quit GMenu2X when window is closed in main screen

This helps when testing on PC.

I didn't bother to implement the same functionality for all dialog
event loops, since that would be a lot of extra work and not necessary
for the scenario that I need it for: clean exit when running under
Valgrind.

src/gmenu2x.cpp

@@ -636,6 +636,9 @@ void GMenu2X::mainLoop() {
 			gotEvent = input.getButton(&button, wait);
 		} while (wait && !gotEvent);
 		if (gotEvent) {
+			if (button == InputManager::QUIT) {
+				break;
+			}
 			for (auto it = layers.rbegin(); it != layers.rend(); ++it) {
 				if ((*it)->handleButtonPress(button)) {
 					break;

src/inputmanager.cpp

@@ -275,6 +275,10 @@ bool InputManager::getButton(Button *button, bool wait) {
 			*button = REPAINT;
 			return true;
 
+		case SDL_QUIT:
+			*button = QUIT;
+			return true;
+
 		default:
 			return false;
 	}

src/inputmanager.h

@@ -58,7 +58,9 @@ public:
 		ACCEPT, CANCEL,
 		ALTLEFT, ALTRIGHT,
 		MENU, SETTINGS,
-		REPAINT,
+		// Events that are not actually buttons:
+		// (not included in BUTTON_TYPE_SIZE)
+		REPAINT, QUIT,
 	};
 	#define BUTTON_TYPE_SIZE 10