Support for Caching entire music library at once

[JackPala]

This could potentially already exist or perhaps could be a good improvement, but the ability to cache entire music folders or the entire library would be super useful for those who live outside of the range of internet frequently, and have larger libraries on Subsonic/Gonic

Modern phones seem to be getting more and more space for less less money, so having a 10-20GB music cache on a 1TB phone would not be problematic storage-wise.