Ultrastar (#26524)

* ultrastardx-beta: init at 1.3.5

* libbass, libbass_fx: init at 24

* ultrastar-creator: init at 2017-04-12

* buildSupport/plugins.nix: add diffPlugins

Helper function to compare expected plugin lists to the found plugins.

* ultrastar-manager: init at 2017-05-24

The plugins are built in their own derivations, speeding up (re-)compilation.
The `diffPlugins` function from `beets` is reused to test for changes in the
plugin list on updates.

* beets: switch to diffPlugins

The function is basically just extracted for better reusability.