Refactor panel grid storage and handling
Refactor panelpage

Panel grid management is now almost completely independent from the
underlyling content framework and could in theory be made even
generic enough to be ported away from being integrated in a cms
Add new panel creation handler
Housekeeping

Remove now unused templates from the content type paradigm version
Fix import handler and add base panel
Add base panel content type
Update block editor header
Add text panel content type
Add text panel content view
Finalize text panel integration
Add layout debugging view with a clear option
Add panel setup and cleanup editor template
Rebuild theme resources and cleanup templates