Bug 1031084 - Save to homescreen is still displayed even though this app...

... is already on the homescreen
tip: in a promise chain, if a rejection was handled, then next `then` will execute. so, this can be written as:
```js
return BookmarksDatabase.get(icon.detail.url).then((bookmark) => {
  this.bookmarkButton.hidden = bookmark ? true : false;
}, () => {
  this.bookmarkButton.hidden = false;
}).then(this._show.bind(this));
```
like in a `finally` block.
see http://www.html5rocks.com/en/tutorials/es6/promises/#toc-error-handling