[Beta] Add flexible download options

[NutchapolSal]

Currently there's only an option to keep 5 latest albums in downloads.
It would be nice to be able to change the selection rule, or add more rules (which would act as OR).
Things I'd like to see would be changing the number of things, and changing the type of thing (so a rule like "100 latest tracks" would be possible).

I would also like an option for changing "latest" to "random" too, but that would probably need more options for specifying how often to refresh (eg. every week, every month) as I wouldn't want the app to constantly download and delete songs every new sync.

Finamp Version: 0.9.11

[NutchapolSal]

just saw this: #101 (comment)

[Chaphasilor]

My plan for this was to have some sort of smart collections, that can contain albums, tracks, and even artists, based on a set of rules.
Then we could use these collections to tell the download system what to download.

But that might require maintaining a local database instead of directly communicating with the server (in online mode), which is not something I want. Because that would mean that data can be outdated even in online mode when connected to the server.

I'm currently busy with other things, but would be happy to accept a PR for such a feature, or even just slightly more flexible rules (like latest X albums).