Update expected return data from plugin search() implementations

[mikkonie]

This should be refactored similar to get_object_link() done in #1343. This is also a good thing to do before #611.

• Return objects instead of arbitrary dicts
  • TBD: Decide how to handle the current dict structure of potentially multiple items
  • UPDATE: It seems the key of the dict is actually never used, so we can simply omit it and return a list of objects
• Deprecate dict return data
• Add separate issue to remove support for dict data in v1.1
• Document

[mikkonie]

Done.