Bug 1026268 - [Vertical Homescreen] Cannot exit edit mode by tapping an empty space 

Does this work if we only check for ```classList.contains('placeholder')```? Checking on all of these individual classes seems a bit fragile.

Also, I know the naming is not ideal, but what if we follow what we do with bookmarks in edit mode? We can tap on a bookmark in edit mode to edit it, and maybe we could also tap on a placeholder to exit edit mode? Just a thought.

Example: https://github.com/mozilla-b2g/gaia/blob/master/shared/elements/gaia_grid/js/items/bookmark.js#L131