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