Support multi folder mode

[maristieuvelonde]

The Subsonic API supports multi folder mode, this is supported by servers like Gonic and would be a wonderful addition.

[eppixx]

I'm using navidrome which chose not to implement folders. Thats the reason it is not implemented (yet).

I'm a bit torn on how the new view should look like. My first instinct is to show columns like some file managers do. Downside is, how to show informations of tracks. Just showing the Title of the track might not be enough to distinguish them and hinders discovery. I might show a slightly bigger element with more information.
When searching I would hide unmatched elements in every column except the selected folders so the file path can still be seen.

I dislike a tree view because it might display the whole folder structure at once.