[Feature request] Cache library information

[el-tiuri]

Not really sure if the title is clear - what I mean to say is keeping a local record/database of the music in the Subsonic library; both the names, artists, … for each song as well as the album art. This would enable browsing of the library without a connection to the server (therefore enabling playing offline music without any connection to the server).

This would also make browsing smoother in general, especially on a slower connection.

A consideration here is that this can require quite a lot of space (especially the album art), so a way to opt out or to remove the local cache might be a good option to have.