Show what markup has been set in the pane preview

This will be postponed for a bit for now.

We'd need to extend the "panels_renderer_editor" class, and make it work with a callback for the style to provide any summary of settings to be rendered , for both regions and panes. Instead of providing our custom renderer class for the edit UI, maybe this can be accepted as a patch for panels itself.

form alter on "panels_panel_context_edit_content" won't really work, because at that point, the output for the regions and panes has been assembled.