IN PROGRESS - issue CORE-4119: Implement simple list editing for App
Integration WYSIWYG property editor 
https://pods.iplantcollaborative.org/jira/browse/CORE-4119

Corrected issue with default argument editor. NPEs were being generated.
Changed how required validation is applied from the App Wizard Field
factory. Now, the user is responsible for applying required validation
via the wizard.

Still need to debug the cause of the validation squiggles which show up
on comboboxes in editing mode. All validators should be removed when in
editing mode.