Independent style settings for slurs and ties

[XiaoMigros]

Resolves: #18769

This PR introduces separate style settings for slurs and ties, with accompanying UI changes to the style dialog.

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• The title of the PR describes the problem it addresses
• Each commit's message describes its purpose and effects, and references the issue it resolves
• If changes are extensive, there is a sequence of easily reviewable commits
• The code in the PR follows the coding rules
• There are no unnecessary changes
• The code compiles and runs on my machine, preferably after each commit individually
• I created a unit test or vtest to verify the changes I made (if applicable)

[cbjeukendrup]

Looks like one MusicXML unit test still desires to be updated...

[XiaoMigros]

The test seems to fail on every mxl test file, but I don't see any issues with my code or the test's code. Do the test files need to be updated?

[cbjeukendrup]

Yes, in this case the test file needs to be updated. (Note that the diff shown in the test output may feel like it's the wrong way around, so the "+" lines are what's currently in the reference file and the "-" lines are what it needs to become.)

[bkunda]

This generally looks fine to me.
If I were to nitpick about a tiny UI issue, there should be a gap between the bottom of the "Slurs & Ties" section and the top of the "Ties" section.

The "System" page has what looks to be roughly 12px of padding here (perhaps this could be used as a reference).

Otherwise it's great! Thanks @XiaoMigros!

[XiaoMigros]

Thanks for the feedback @bkunda! The layout now matches that of other pages in the dialog.

I also changed style.cpp to automatically adopt the slur values for ties as well, when importing older scores. This means the vtests are now successful, and the changes made to the MXL test files have been reverted (@cbjeukendrup).

[bkunda]

I've given this another test on macOS.

Only thing I noticed was that modifying settings for Slurs seemed to affect my ties as well. At the same time, modifying "Line thickness middle" for ties didn't have any affect at all. See video

[cbjeukendrup]

Another rebase is needed since the recent work in the MusicXML module...

[cbjeukendrup]

From the combination of Bradley's latest comment and Simon's approval after the last fix, I assume that this is ready for merging, right?