Added missing shortcut for menu bar toggling

[sdrwtf]

I have added a shortcut and menu entry which allows to toggle the menu bar by pressing F11. It calls the already existing function in dekstop/main.js (line 63):

app.toggleMenubar = function () {
  app.win.setMenuBarVisibility(!app.win.isMenuBarVisible())
}

This might resolve #145

Menu entry:

Toggle by mouse-tap and pressing F11:

[waldmannly]

This is cool! Not sure I love it being bound to F11, as that is usually full screen for most programs.