Added work relationship to batch add relationships.

This is going to be important for linking to books, as required by the new audiobook style guideline. The original ticket also gave linking recordings on a "single" release as a use case.

Enabled linking of work relationships using the "Batch-add a relationship to recordings" button, in addition to the "Add related work" button. This has the side effect of enabling the type selection box on the "Add related work" dialog - when clicked - this has a single value - work.