Add the pane preview functionality from the panopoly magic module

Panopoly is a great distribution that really shows off what panels can do together with panelizer. It has a module, Panopoly magic, that greatly enhances the pane selection and edit form, which improves the user experience greatly. Having a separate module is great, but I think we should try to merge these great improvements into panels, so that everyone can enjoy them.

I will attempt to create a patch for the UI improvements in the pane selection and edit form based on the panopoly here, and see if we can merge it into the actual panels module.