Define MonadClientUIFrontend monad that communicates with the frontend

[Mikolaj]

Then the current MonadClientUI can only read and write to UI session state (and read form client and main game states) and probably most of the code will remain in MonadClientUI, making it easier to test and perhaps suggesting refactorings that minimize MonadClientUIFrontend usage.

Bonus points if liftIO can be removed from MonadClientRead after this change, so that mocking is easier and more obviously safe.

[Mikolaj]

I'm surprised this is not done already, so there may be some good reasons I forgot. But probably the MonadClientUI code was not as large and nasty last time I considered that.

[Edit: possibly, almost everywhere the frontend is called directly or indirectly. If so, that's a challenge and the new monad class will help monitor success in unravelling that. Getting rid of liftIO will be useful regardless.]

[Edit2: no, it's not so bad, e.g., all the messages-related code does not touch the frontend.]