Playback menu: Add "Play/Pause" item

The goal here is to remove shortcut handling from the renderer and
unify it all in menu.js and shortcuts.ks (for alternate shortcuts).

I would rather name it "Play" and change to "Pause" when video is
playing, but Electron doesn't support this (yet).

renderer/index.js

@@ -1155,8 +1155,6 @@ function onKeyDown (e) {
     } else {
       dispatch('back')
     }
-  } else if (e.which === 32) { /* spacebar pauses or plays the video */
-    dispatch('playPause')
   }
 }