Don't parse autostart files n-phase times

Summary: Load only once and share

Test Plan: Restarted. Also made a small test app that I'll submit once this is in as it depends.

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: davidedmundson, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D23719