Add a new plugin API extension point (register_metadata_tag_action) that allows plugins to register context menu actions for metadata tags. Also adds a title parameter to register_script_variable so plugins can provide display names for custom tags in the metadata box.

New API:
  

• MetadataTagAction base class (inherits HasDisplayTitle)
• PluginApi.register_metadata_tag_action(action_class)
• callback(self, tags, objects) receives selected tags + objects
• is_visible(self, tags, objects) controls menu visibility
    
  Enhanced register_script_variable:
    
   - New title parameter for metadata box display name
   - Refactored internal storage to PluginVariable dataclass
    

Example plugin: https://github.com/zas/picard-plugin-pinned-tags

 

Based on: https://community.metabrainz.org/t/artist-folder-naming-how-do-you-handle-artists-with-identical-names/814857/14?u=zas