Make KColorScheme compatible with QVariant

Summary:
This requires default constructability and a meta type declaration. The
use-case for this is to expose a KColorScheme instance to Grantlee.

Reviewers: dfaure

Reviewed By: dfaure

Subscribers: dfaure, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D24060