Improve UX of saving custom tunings in 'Tunings & temperaments' plugin

[avvvvve]

Follow up from: #28709

Implement improvements to saving over tuning presets and renaming custom tunings. Details in this comment: #28709 (comment)

[XiaoMigros]

If someone has time to work on this, let me know and I'll share my development branch with my current progress!