restore the "current" color scheme concept

Summary:
add again a fake entry called current like there used to be,
add an "Apply" button in the color scheme editor (when not in
standalone app mode) hitting apply will apply whatever custom
set without saving it, will be accessible trough the "current"
entry.

Test Plan: changed a single color in the set, gets correctly applied

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: davidedmundson, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D5411