Add ability to download all songs the current playlist

[eharris]

I use the server shuffle mode a lot, and it would be really nice if there were a way to tell iSub to download/cache all the tracks in the current playlist. That way I could pre-cache a couple hours of music when I'm going to be off wifi for a while without eating up my cell data.

Maybe this could be added as an additional option for the "Edit" mode, so that selected songs (or the entire playlist) can be marked to "Download" instead of only "Remove".