Rethink schema

Before finalising 0.1 release, we need to see if there are improvements to the scheme.

For example, can we get rid of `show_actions` and just allow creating the entire component dynamically with `evaluate()`, thus opening up a lot more customisation options.
https://github.com/aio-libs/aiohttp-admin/blob/d8ab4623879bf66a35cd994a82e54f882b452367/aiohttp_admin/types.py#L93-L94