Add API and prefer it over websockets where possible

[jojomatik]

Is your feature request related to a problem? Please describe.
Currently all information is sent over websocket connections. This works fine but makes truly async implementations of request and response far more complicated. Also I don't think that websockets are not designed to be used this way.

Describe the solution you'd like
Add an API that provides similar information as the current websocket implementations. Use API for initial requests and changes sent from the frontend to the backend. Use websockets for real time updates. Both implementations should use the same types etc. for consistency.

Describe alternatives you've considered
None. The API should be part of 1.0.0 anyways, the implementation in the frontend code should be done at the same time in my opinion.

Additional context
None.