Symlink artwork in SymlinkViews?

[wisp3rwind]

As mentioned in https://github.com/geigerzaehler/beets-alternatives/pull/48#discussion_r396076268, we could consider linking artwork in SymlinkViews, which would be interesting for users which do not employ the embedart plugin on their main library. Directly embedding the artwork ourselves (as the embed option does in the case of External collections) is not a sensible thing since it modifies the original files.