Include writable part names for Staves from the plug-in interface

Automatically generated content is often dependant on other staves and only one instance needs to exist because of this dependency. When users modify content and want to regenerate automatically generated content (such as the two "guide-tone" melodies from a chord progression), the old can be overwritten by the new.

Using customised part names could be used to help a plug-in keep track of these staves.