Revision API of catalog.

[polina-c]

It should be possible to modify different aspects of catalog items (provided by genui or third party), while still taking advantage of the rest.

And, these aspects of catalog items should be re-definable separately and observable:

1. Item definition schema / object
2. User input schema / object
3. State of the item
4. Item rendering
5. User input handling

Related issues and discussions:
#528
#609