Proposal: Transcript voice selection rework

[eteubert]

Goals

• by default, if no contributor is assigned/selected, the transcript still works by defaulting to the voice string from the vtt file (instead of an empty transcript)
• add a dedicated option to hide a voice from a transcript

Notes

• I would usually use something like -1 for special cases but since contributor_id is an unsigned int, I use 4294967295 -- which should at least be turned into a constant in the final implementation for readability
• Think about how this change affects users: Not selecting a contributor is not an intended feature, but people may be using it to hide contributors. After this update they might then have unwanted voices appearing in their transcripts.