support custom conditions

[simon-weber]

Discussed in this mailing list post, custom conditions would basically be autoplaylists with some behavior changes:

• they don't sync
• limit not capped at 1k
• can be converted to and from a normal autoplaylist
• show up in a different list in the ui
• don't display in autoplaylist linking condition
• display under separate header? not sure what the UI should look like

[vokinpirks]

I wanted to suggest the same feature. I like "specifications" concept.