Use kid3 for tag getting/setting

[Chris Warburton]

It seems to support everything, although kid3-cli is a little awkward:

 - We have to give the directory name, and run a command to "select" the
   file
 - We have to quote everything carefully, since we're building up
   kid3-cli commands inside bash commands
 - We have to run "save"
 - We need to set DISPLAY, even though it's a CLI...

All in all, a bit of a faff.

We have a "set tag" script. We should make that check for kid3 and fall
back to nix-shell if not found; that way, we can provide kid3 from
another script, and not have to run nix-shell over and over in a loop.

We should make a "get tag" script.

Edit the other scripts to use these for their getting and setting needs.