Add new API functions to support browsing the collection by genre/artist/album

[agersant]

This task spawned from user requests (agersant/polaris-web#58, agersant/polaris-web#67). Both requests asked for the Polaris web client to use song metadata in order to clean/organize the user experience, regardless of the quality of the file names and organization of the collection.

The goal is for clients to be able to implement an interface similar to Columns UI, filtering music by artist, album and genre.

Tentative new endpoints:

• Get the collection structure as a tree of genres -> artists -> albums
• List songs matching a set of filters allowing a subset of genres, artists and albums. (eg. list all Pop songs, list all Rock or Jazz songs by R.E.M or Casiopea, list all Stratovarius songs on Destiny or Episode)

Special attention should be given to the following details:

• Large lists of songs may result in very big payloads, which should be compressed (Gzip Compression & Browser Caching rwf2/Rocket#195)
• Songs with missing metadata (genre, artist, album) should be queriable when there is no corresponding filter
• Artists who only appear on songs that they are not the album artist for should not be queriable
• Sorting should always prioritize metadata over file system organization (eg. sort albums by year, sort songs by disc/track number)
• If possible, albums by multiple artists which are not correctly tagged with Album Artist metadata should not appear as duplicate albums by the individual artists
• The same artist may appear in multiple genres
• Each song may have multiple genres
• Each song/album may have multiple artists (overview of parsing strategies for ID3v2)

[agersant]

More fields may be good to consider for this feature, see this reddit thread.