Support separate controller grouping keys and display labels

[caseyanderson]

Performance layouts currently use source names and Spectra labels as grouping keys. This requires visible controller labels to match the source name exactly.

Add a way to configure these independently:

• A stable source/group key used to associate Twister controls, Spectra buttons, and performance gain-stage content
• A short visible source label for controller headings
• A separate visible action label for additional Spectra buttons associated with the same source

The existing behavior should remain the default for projects that use one source name for everything.