possible to edit the default color scheme

Summary:
no reason to not allow editing the default color sheme
with an invalid path, everyting falls back to the default,
keeping the dialog fully functional

Test Plan: edited default color scheme, saved as a new one

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D5406