New features and improvements for fretboard diagrams

[mike-spa]

A big revamp of our fretboard diagrams, including

• A completely redesigned style dialog (some options are different, some are new)
• New option for slur-styled barré
• New option for custom fret number suffix
• New option for trimmed-style (default) or extended-style diagrams
• New option for entering and displaying fingering (with related UI components to edit them in property panel)
• Many small engraving improvements and corrections.

Note: the underlying code is really not in a good state. The drawing code is especially problematic, with lots of repeated code, and especially lots of stuff that shouldn't be there at all but should be moved to the layout code, but I don't have the time at the moment to fix it all, so I did little pieces where I could. We'll have to come back to it and tidy it up properly.

[oktophonie]

Permit me to add:

1. Use 'Barré' (with the diacritic) in the Style dialog
2. Where there is a slur-style barré, dots should not be shown at the barré position where that string has a dot at another position (or, conversely: dots should be shown if there are no dots at another position). e.g:

[mike-spa]

@avvvvve

1. ✅ Fixed
2. ✅ Fixed
3. ✅ Fixed
4. ❌ I couldnt' figure this one out. Also because, at least on my machine, it doesn't always happen. And I still haven't understood exactly which conditions cause it to happen. I may have to enlist help.
5. ✅ Fixed (this was just an update issue: you would have found the additional fingering element by closing and reopening the panel)
6. ✅ Fixed
7. ✅ Fixed
8. ✅ Fixed
9. ❌ I disagree on this one. The diagram is normally entered above a note and is vertically centered with the notehead, and it's important that it stays centered at any magnification level. I don't think you'd normally write a diagram on an empty bar. In this case it is just centering itself on "where the first note would be" if the bar had any note. @oktophonie do you think it's worth adding some special case here for diagrams on an empty bar (e.g. centering them in the bar?)
10. ❌ Can't reproduce it on my machine (and it's very likely a problem that predates this PR, so it's not to be fixed here)
11. ✅ Fixed
12. ✅ Fixed

@oktophonie

1. ✅ Fixed
2. ✅ Fixed