Plasma change icons should exit after its work finished.
Right now plasma-iconchanges may remains on the system after change the icon
Test Plan: Run plasma-changeicons manually. It does exit and change the icon theme.
Reviewers: apol, mart, Plasma, broulik
Reviewed By: Plasma, broulik
Subscribers: broulik, plasma-devel
Differential Revision: https://phabricator.kde.org/D8529