[KColorSchemeManager] Don't list duplicates

Summary:
Loading code taken from the colors KCM. Additionally it ensures that when color
schemes with the same file name exist in multiple categories the right one is
loaded.

Reviewers: Frameworks, ngraham

Reviewed By: ngraham

Subscribers: ngraham, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D26948