Set tab order for RGB sliders & spinners in OPIColorDialog

This commit sets the tab order for the RGB sliders and spinners in `OPIColorDialog` to: red/green/blue sliders; followed by red/green/blue spinners.

This means that if you type a value into the red spinner, pressing Tab takes you to to the green spinner.

Chances are that if you've typed in one of the red/green/blue values, you want to type in the others; this makes it easier to do that.