Add support for allowing the Music Release category slug to be changed

[thomasmery]

At the moment the plugin needs a Music Release WC Product category to function.

This category can be renamed by a User but its slug can not be changed

We should allow the category slug to be changed.

This could go as follows:

• set a custom option with the slug of the category used to categorize Music Releases
• use this slug (as obtained from the option) everywhere the Music Release category needs to be used
• detect when the user changes the category slug and update the option accordingly