Provide config schema for Entity form displays

This is a sub-issue of #1910624: [META] Introduce and complete configuration schemas in all of core.

Problem/motivation

#1866610: Introduce Kwalify-inspired schema format for configuration introduced some config schema coverage for views, but it is not complete. The changelog leads to (hopefully extensive) documentation on the format at http://drupal.org/node/1905070. While there are little cleanups planned for the format overall, the current format is a result of months of back and forths, so it should be perfectly fine to apply it more widely to core.

Proposed solution

Figure out the missing pieces that are not yet covered. Write schema file sections for them. Clean up / fix any issues in current schema.

Create a configuration schema for entity type entity_form_display.