Expose element links (score<->parts) to plugin API

I would like to be able to access elements in the score from their equivalent in the part like so:
-user adds an bracket style to an accidental in the part
-user selects that accidental
-user runs the plugin
-accidental in the score then reflects that bracket style

I know there are requests to be able to selectively link element properties between the score and parts, but this would enable plugins to get at that.

See comment: https://musescore.org/en/node/331097#comment-1124848