Add fragment selection for element-type links

[engram-design]

There are times where you might want to link to a fragment on a page - for example linking to an ID on a page.

You might have: <h2 id="my-heading">Example Heading</h2> and you'd want to provide a nice means to link to that. This would only be possible for element-type links, and not 100% sure on the UI for this, combining with element select fields. But you would select the element you want, an Ajax request to query the front-end template for that element, parse just items with an ID attribute, and list those for picking.