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