kcms/colors: Small refactor to simplify bindings and interaction
Some small simplification and clean-up of the kcms/colors code I did while trying to do !1748 (merged).
In the end these weren't necessary to fix the issue but here they are anyway
- To set the wallpaper accent color, use the property setter instead of an invokable, so we reduce one codepath and it's easier to keep the state in sync between UI and backend.
- Reduce unnecesary explicit bindings on the QML side
- Apply clang-format
- Rename property to not look like a verb
- Use new connection syntax