Implementing presentation edits

[booski]

It should be possible to make some "deeper" edits to an existing presentation, as in edits that need to be sent to the storage backend for processing. The things that should be editable like this are:

• Thumbnail
• Posters
• Subtitles
• Streams:
  • Add
  • Remove
  • Replace

The daemon has been updated to support all of this now, documentation for how it works is here: https://gitea.dsv.su.se/DMC/play-daemon (specifically under the headings "Incoming" and "Update").