Switch to Qt 6.8

[cbjeukendrup]

Resolves: #11244
Resolves: #12467 (probably, according to comment there)
Resolves: #22416
Resolves: #23505
Resolves: #23970
Resolves: #24206
Resolves: #24232
Resolves: #24579
Resolves: #24613
Resolves: #24667
Resolves: #24716
Resolves: #25300
Resolves: #25307
Resolves: #25379

[cbjeukendrup]

Status update:

• on macOS, it works
• on Linux, I can't check but I think it works
• on Windows, CI builds (almost) work, but local builds not, because they can't find Qt DLLs.
  • There are two solutions:
    • letting everyone add Qt library folders to their PATH (for normal targets, Qt Creator does this automatically, but not for "Custom executable" targets; and we have to use the latter, because we always need to use the executable from the install folder)
    • making windeployqt part of the CMake install step (might take long-ish)
      I'm not sure yet which one I'll choose.
• we finally have VTests to look at!
  • I see no major disasters at a first glance
  • It looks like they refined the BBox calculation for text, seemingly for the better. This causes some texts to land on a different pixel.
    • At lyrics-9-1, you can see that the bbox is now less overestimated.
    • It is beneficial for center/right alignment, see frametext-1.
    • Sometimes, this causes slightly more drastic layout changes, because now some items suddenly fit 'before' something else instead of trying to avoid it by moving up vertically. See for example musejazz-10-1.
  • There are some disappearing texts though:
    • the 'full' text on (old?) bends (bend-1-1)
    • the fret number for fretboard diagrams (harmony-align-1)
    • (in lyrics-11-1, the gliss text appears, because with the new bbox calculation there is now enough space)
• And then we have the fact that the version of KDDockWidgets that we use is not really compatible with Qt 6.5+, see Qt 6.5+: docks do not open after the last dock on a panel is closed #24866
• And we'll doubtlessly find more bugs. I already found two:
  • on Windows, the text on the Spash Screen seems to be partially missing
  • the positioning on those big icon+text buttons looks off
    

Next thing I'm going to try is what happens when we switch to DirectWrite.