fix #175886 fretboard to harmony

[MarcSabatella]

This is a 2.2 version of the PR I submitted for master - #3199. I still need to add another mtest to this, and to update the master version as well.

[MarcSabatella]

OK, I think this is ready. IWill now go back to my original PR from master and update that as well. Same PR can't work for both as too many things have changed.

The intent of the changes here is to make fretboard diagrams work more like chord symbols and to make it easier to work with them together. The specific changes:

1. you can now add a fretboard diagram to a chord symbol rather than just to a note or rest, via drag &
   drop or select / double click (internally it still gets added to the segment)

2. you can now add a chord symbol to a fretboard diagram via Ctrl+K (again, internally it still gets added to the segment)

3. you can now copy/paste a list selection of fretboard diagrams same as for chord symbols

The tests added cover the first and third points. The second I don't know that there is a way to test automatically.

[MarcSabatella]

#3211 should address the most important concerns I have - bugs that were preventing this code from working as it should. Chord symbols seem broken in other ways too but not relevant to this feature.

NOTE: for future reference, the above comment about bugs preventing this actually applies to master (#3199), not this PR which is for 2.2.