partial fix for #23324: links for section breaks, frames, & fret diagrams

[MarcSabatella]

Links for section breaks (present before parts created), frames (added after parts created) and fret diagrams (both before and after).

[MarcSabatella]

The Travis failures are the result of my changes, and I think they are correct, but not having Guitar Pro or understanding how it works, I can't say for sure if the new behavior is correct behavior or not. FWIW, I elected to include fret diagrams in the "copy to linked parts, but not to linked staves in same score" category. Meaning, if you have a score that includes a guitar instrument with two linked staves, the fret diagram will appear on only one of the staves (just like chord symbols and a handful of other element types), but will appear on the extracted part as well (again, just one the corresponding staff).

[jpirie]

I'll have a peek at the resulting score now, I have various versions of Guitar Pro I can check this against.

[jpirie]

Looks great to me! The failing guitar pro tests for fret diagrams should just be updated with new solutions. (This issue actually tangentially touches on the discussion I'm having on the mailing list about which staffs certain items (mailing list discussion is focused on fingering) are placed on, perhaps some users may wish this laid out only on the tab staff/only on standard notation staff/both on standard notation and tab staffs - might be nice to think about this at some point in the future.)

[MarcSabatella]

Thanks! I'll update the tests later todasy.

BTW, there are two places in the code where the decisions are made about which items to duplicate and which not - in excerpt.cpp where the parts are created (if the elements exist in score already) and also in undo.cpp where elements are added (if parts have already been created). You'll see that's where I added FRET_DIAGRAM in my code here. Right now, we just just make an arbitrary decision based on element type as to which items not to duplicate, but it would presumably be a simple matter to add some sort of flag - either on a per-element basis or perhaps via a style or staff type option - to control this further. It just hasn't been a priority.

[MarcSabatella]

Now it's just a general Travis error. I updated the tests and they pass for me locally.