'Add performance' not available at work level if work has multiple versions

[axgeertinger]

Currently, performances cannot be added at work level if the work has more than one version. Until we have fixed this there are two workarounds:

1. If possible, enter performance data before creating the second version of the work. Once there is at least one performance at work level it is possible to add more performances later.
2. If versions are created already: Enter the performance data under one of the versions, save it, and use the perfromance's 'Move to...' option to move it to 'Work (no specific version)'.

[axgeertinger]

fixed