heuristically find the color scheme

Summary:
why this loop trough all the scheme files?
the scheme theme name is an heuristic, there is no plugin metadata whatsoever.
is based on the file name stripped from weird characters or the
eventual id- prefix store.kde.org puts, so we can just find a
theme that ends as the specified name

Test Plan:
now themes downloaded from store.kde.org are correctly applied
such as "Kshell" use case

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D4066