Summary: port the shutdown dialog to package::fileUrl, dropping the support for old custom themes

Test Plan: shutdown dialog work correctly both with normal and rcc kpackages

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: broulik, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D9195