ColorUtils: Use const ref for QColor/QJSValue, fix code style

Summary:
Use const ref for QColor and QJSValue values.
Clean up code style, lots of missing braces and spaces.
Don't use std::map if no lookup by key actually happens.
std::vector is a much less heavy-weight container.

Test Plan: Things still work.

Reviewers: Kirigami, mart

Reviewed By: Kirigami, mart

Subscribers: plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D29046