ColorChooser : Hollow circle indicator

[ericmehl]

This changes the indicator for the color field and sliders to a white circle, with an inner and outer black stroke. The previous indicator was a solid blue circle, which covered the exact value. I tried a white circle with just an outer black stroke, but it felt like it read a little less clearly in some areas of grayscale, but light and dark.

I'm happy for suggestions on a better API for setting the colors and shapes of the indicator in the second commit. I tried using the hollow circle for all sliders, which currently means the spline slider. It was less visible and it felt less inviting to click and drag on than when used in the color sliders.

Checklist

• I have read the contribution guidelines.
• I have updated the documentation, if applicable.
• I have tested my change(s) in the test suite, and added new test cases where necessary.
• My code follows the Gaffer project's prevailing coding style and conventions.

[ericmehl]

Based on an offline conversation, I reworked the original commits. Instead of adding more protected methods to override, I duplicated the relevant code for drawing the value from GafferUI.Slider into the color chooser sliders. I also changed the indicator to be a black circle with a white circle inside it (both unfilled).

Ready for a review.