Simple declarative store to describe custom buttons.
Plus universal handler to handle any button depending on its type.

Main stores are covered with some popular props and methods:
AppStore, AnnotationStore, Annotation

They will fully substitute all other buttons for now.
Later we'll add some other buttons specified in custom buttons config.

This allows to mix custom buttons with internal ones.

- look for Submit is primary as a fallback value, because it has no secondary
- Button now have a `mod` property from Block, should be infered, but that's for later
- Dropdown pointing to a correct component with additional property
- the rest are just added types or direct type casts