GUI RealTime activity: misplaced spinbox in parameter tuning line

At some point during my recent GUI refactoring, the spinbox in parameter tuning line got misplaced: it now has an unwanted margin on top and on the left.

For analyzing the problem, I gave the spinbox a gray background, and added a thick border to the entire tuning widget (spinbox + slider). These must be removed when the issue is solved.

The tuning widget is created in function ParameterTuningDelegate::createEditor.