#97375 More Panels 2 code cleanup and move non-content module Panels integration to relevant module - fieldgroup, nodereference, and userreference.

#97375 add fields to the panels 2 integration, and misc code cleanup, patch by gordon, KarenS, and radiantflow.

#203495 followup - fieldgroup panel content : fix groups weight being displayed, and better code for rendering the group's fields

#203495 folowup - typo sneaked in while refactoring Michelle's original patch. Doh. I seem to keep doing this lately...

+ make group selection a select instead of radios buttons

#203495 followup - check that fieldgroup.module is actually enabled, or we'll face some 'undefined function' calls...