E.showMenu: watches on BTN's stay active when menu is left

Hi!

The issue is that the menu likes to stay around until it is explicitly removed - there are a few cases where it's handy.

However all you need to do is call E.showMenu(); to remove the menu:

var mainMenu = {
  "": { "title": "Menu" },
  "Start a game": () => {
     E.showMenu();  // <------------------------ this
     startGame()
   },
  "Exit" : () => closeApp(),
};