Enhancements to preview content before saving it.

Currently it supports only node content editing. But the idea is to support more preview functionality by abstracting, and offering, the same preview functionality as the node module in drupal core does.

What it does is using the preview route as a src for an iframe to show through the off canvas dialog. Altering the existing preview button to do an ajax submit and trigger the off canvas dialog. Making it possible to show it in your editing screen.