Use % for tuplet 'Maximum slope' style setting

[XiaoMigros]

The 'Maximum value' value is currently falsely displayed as a spatium value in the UI, even though internally it's treated as a double.

• I signed the CLA
• 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)

[ghost]

Might it not be even nicer to express this in degrees (horizontal being 0°), perhaps?

[XiaoMigros]

The difficulty with using degrees is that we would encounter issues using the arrow handles to change the value, as internally the value is stored as a slope. Unless we apply a clever conversion (which would involve more intrusive code changes to the already messy dialog), the step sizes would be result inconsistent, seemingly random numbers.
The other solution would be to change the internal engraving system to use degrees, and apply a conversion when importing older scores. If I could choose, that would be my preferred solution, since using degrees makes more sense. That would mean a couple of lines of added code in the style import code, but leave the dialog pretty much untouched.

[bkunda]

@XiaoMigros I think we're in agreement that the % is better than the spatium value. At this stage, I'm not sure it's worth the effort to try and adopt degrees (although this is obviously the most correct option) given the issues it would present for maintaining compatibility.

@cbjeukendrup will give it a quick rebase so we can test it. Many thanks!