Cache for songs

[X-Ryl669]

Currently, unless using LocalStorage, it does not seem possible to cache songs data (and local storage is too limited to be of any use anyway for this). None of the resolver seems to cache the songs' data.
Does Tomahawk does it on its own ?
If not, what would be a cross platform way to cache songs data once downloaded ?
Could we add something in the resolver for this (and if yes, what should we add to the player for this to work ?)

Just for reference, having a cache in Tomahawk player itself was ruled out by @muesli in #569:

muesli commented on 20 Oct

We'd love to support that, but it's obviously a legal minefield. Some plugins like Spotify do their own internal caching though.