Panelizer field list all possible view modes

This needs tests to confirm the change works as intended.

This is also a duplicate of #2803089: Panelizer field should only show the enabled view modes, but it doesn't have a patch yet.

Thanks for the fix! Committed.

This patch broke my site at edit time after updating panelizer to 4.1.0. For anyone who's getting notices like the following on panelized node edit pages, you may want to avoid pressing "save", or you may lose your custom overrides for a given view mode that you've entered by the IPE on a given node:

Notice: Undefined index: full in Drupal\panelizer\Plugin\Field\FieldWidget\PanelizerWidget->formElement() (line 112 of modules\contrib\panelizer\src\Plugin\Field\FieldWidget\PanelizerWidget.php). 

Warning: Invalid argument supplied for foreach() in Drupal\panelizer\Plugin\Field\FieldWidget\PanelizerWidget->formElement() (line 112 of modules\contrib\panelizer\src\Plugin\Field\FieldWidget\PanelizerWidget.php).

For me, the fix was to add "Full content" to the custom display settings on the content type display screen and to manually delete the rows in node__panelizer corresponding to displays that weren't enabled and clear cache. SQL I used, below, but be careful that you don't kill your website. :)

DELETE FROM node__panelizer WHERE bundle = 'page' AND panelizer_view_mode NOT IN ('default', 'full', 'header', 'teaser');