Components validation

[jamowei]

If added a first draft of component validation. Please review!
Take a look here for running example.

[ghost]

Looks quite nice to me!

I believe you prefered to remain flexible with the type of the store by relying on runtime type checking within the fun <D> Store<D>.validationMessage(): Flow<String> function instead of defining some ValidateableStore, which all of our components then would have to adopt, even if there is no need of validation? (A possible typed variant would require some kind of null pattern for the validator in order to remain kinda optional though.)

I would like to tinker a small example for our formControl component in order to get a feeling how this approach fits there. My initial guess is, that this would work very good, as the store is injected into the underlying sub component, but the validation is handled by the formControl itself. With this kinda dynamic approach one could try to validate but catch the RootStoreIsNotValidatable if validation is not possible. Thus the user is not forced to implement validation if not needed.

[jamowei]

Ok, I have done the rework. Please review again and if there is noting left to do, I will start documenting.
@jwstegemann @chausknecht