Search for / handle unavailable library tracks [Implemented]

Feels like an amalgamation of these two requests (along with the ability to also identify missing albums) would cover most bases here.
https://community.roonlabs.com/t/select-search-unavailable-tracks-in-playlists/110664

Agree the ability to identify a replacement track at the point of playback (or to flag them so they could be identified/reconsolidated later) would also be useful.