[KColorScheme/KStatefulBrush] Switch hardcoded numbers for enum items

Summary: This also replaces some for-loops with C++11 range based for-loops and switches for simpler if/else control blocks.

Test Plan: Open colorscheme editor in the Colors KCM to see if any colors or effects look broken.

Reviewers: Frameworks, dfaure

Reviewed By: dfaure

Subscribers: ngraham, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D25677