Combine Speakers with same name, disallow empty names

When changing the name of a speaker to a name already used, this should instead change the speaker to the one which already has the name.

Also the change speaker name modal should not allow an empty name

Start at the change speaker name modal and trace the speaker rename flow. Check how duplicate names and empty input are currently handled; done means renaming to an existing name selects or combines with that speaker and empty names are rejected.

Written by the indexing model from the issue text.