FileMergeDialog has bad layout
That went better than I expected. I haven't worked on Qt layout code in years, but I think that's about right. I didn't overthink it, and it might not have been a good approach, but the dialog is legible now.
Fix #1590.
Update copyright to 2021
FileMergeDialog has bad layout
Spanish, American, British translations
I'm getting an average of 7 seconds per run using the previous testing procedure. That's up 3 seconds from the 2017 version I tested, but it's vastly better than the 44+ seconds I was getting with the prior revision of SVN. It seems acceptable. This particular file is pretty extremely ridiculous anyway, and I bet Rosegarden performs even better in the real world. I doubt we see too many real compositions running over 10 minutes spread across 480 segments.
Contribution from Arthur Baran