Move the media player functionality into separate plugin. This requires extensions to the plugin system.

Rationale: The player is based on QtMultimedia, and as such heavily depends on the playback capability of the underlying platform. While it works quite well in general, it means not all formats supported by Picard can be played.

Having this as a plugin would also allow for alternative implementations which could use different playback backend implementations.

Audio playback also brings with it a whole set of additional complexity that makes it difficult to support fully as part of core Picard.