Create panel pane with configuration form

Problem/Motivation

Unfortunately, block configuration panes are not available in Panels per #1119862: Block op = configure form items missing from pane config → .

Proposed resolution

Create custom panes plugins with their own configuration forms.

Remaining tasks

Initial commit of code for testing.

User interface changes

Will be available in Panels. No changes to the administrative form have been planned.

API changes

N/A

Data model changes

N/A