Added love.window.showMessageBox.

It has two variants: showMessageBox(type, title, message [, attachtowindow=true]), and showMessageBox(type, title, message, buttons [, attachtowindow=true]). The former shows a simple message box with a single OK button, and the latter displays several buttons. The buttons argument is an array of button names.

The second variant returns the index of the pressed button, or 0 if the message box was closed some other way. showMessageBox will block until the message box is closed (and it can't be called on a separate thread.)

src/modules/window/wrap_Window.h

@@ -50,6 +50,7 @@ int w_hasMouseFocus(lua_State *L);
 int w_isVisible(lua_State *L);
 int w_getPixelScale(lua_State *L);
 int w_minimize(lua_State *L);
+int w_showMessageBox(lua_State *L);
 extern "C" LOVE_EXPORT int luaopen_love_window(lua_State *L);
 
 } // window