Plasma change icons should exit after its work finished.

Summary:
Right now plasma-iconchanges may remains on the system after change the icon
theme.

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

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D8529