Fix Control Center metadata sometimes disappearing after item deletion

[defagos]

Description

This PR fixes an issue with Control Center metadata sometimes disappearing after item deletion, e.g. after reshuffling the item after the current one with another down in a playlist, then deleting the current item.

The issue was located in our ReplaySubject implementation (similar implementations like CombineExt or Entwine have the same issue). See #785 for an explanation of the issue and how to solve it.

Changes made

• Fix ReplaySubject implementation to support in-order value processing in multicast configurations where some subscriber might trigger a new publisher emission on the same thread.
• Make ReplaySubject public (just an oversight).
• Extracted QueuePlayer-related types into separate files (general improvement).

Checklist

• APIs have been properly documented (if relevant).
• The documentation has been updated (if relevant).
• New unit tests have been written (if relevant).
• The demo has been updated (if relevant).