Add "Characters" CRUD screen

[dafrie]

We decided, that we want to have a central place to store characters, were new characters can be added or renamed. They will belong to a "book" and should not be able to be deleted until no "bubble" (in a dialog) are using this character anymore.

[dafrie]

@YanickSchraner: Just realized: Do we even still have characters in the second book? Because it seems that got intentionally "lost" in the concept of the second book. At least I do not really see a pattern with names. Certainly not the same as the first book

[YanickSchraner]

@dafrie I'll check this with the "Redaktionsteam" and update the issue accordingly.