fix "Default" color scheme

Summary:
for every color scheme entry, use an actual KColorscheme instance
that loads the kconfiggroup representing the color scheme
this way, it will correctly fallback to the color scheme defined as
fallback in the kcolorscheme implementation

Test Plan: changed color schemes back and forth, default is same as breeze, decos included

Reviewers: #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D3053