Add number of classes to dataset / training

[plieningerweb]

Issue?
See #1384
People try to add a custom dataset, but do not recognize that their only class is 0 -> DontCare.
This is really hard to debug / bad UI, because the user can never see / double check, how many labels / classes there are and are used during model training.

Suggestion:
Show in the dataset details a button to show distribution of labels or, if not possible, then in the model training overview page.