Add static method to check start condition

Summary: Needed for systemd autostart generator will add a ExecCondition=kde-systemd-start-condition rcfile:section:entry:default, as described in T12627

Reviewers: davidedmundson, broulik, dfaure, kossebau

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D28421