Add ability to get path to a score from plugins

[ecstrema]

Resolves: no issue in tracker I know of but a forum discussion: https://musescore.org/en/node/314074

When for example saving a copy of a score, one might want to know where the file was initially read from.

NOTE: I still cannot build on my PC, so I'll test the artifacts when they get generated.
UPDATE: I have tested the artifacts and everything to worked.

will need a master counterpart

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