Color Selector Uses Theme Color for Background

This video can explain it quite nicely! advancedColorSelector-2020-06-06_18.40.03

I personally didn't quite like how the color selector didn't match with the overall theme. For this I created a new option in the Color Selector Settings menu to allow for the background color to be the same as the theme. It also correctly deals with changing theme, and the different shade selector methods.

This behavior also applies to the Show Color Selector pop-up, without errors.

Test Plan

1. Open the Advanced Color Selector Settings menu in the Advanced Color Selector Docker.
2. Change the background color setting, and verify that it indeed changes.
3. Change theme and verify that it updates.

Formalities Checklist

• I confirmed this builds.
• I confirmed Krita ran and the relevant functions work.
• I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
• I made sure my commits build individually and have good descriptions as per KDE guidelines.
• I made sure my code conforms to the standards set in the HACKING file.
• I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.