Pixel spacing SpinBox drag operation broken

The pixel spacing SpinBox is a StudioControls.RealSpinBox and not a HelperWidgets.SpinBox, because it isn't connected to a backendValue. Right now the drag operation logic is implemented in the HelperWidgets.SpinBox, because it sets the value directly on the backendValue which isn't available in the StudiControls.RealSpinBox.
To fix this issue this logic needs to be transferred to the StudioControls.RealSpinBox.