KCM Icons fix theme selected when we hit delete theme

Summary: CUrrently when we delete a theme the next one is selected. With this fix we stay on the currently selected one and to simplify code don't allow to delete the currently selected theme

Reviewers: mart, ervin, Plasma, crossi, ngraham, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: bport, ngraham, broulik, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D24847