Import KAutostart class from KService
The handling of the autostart is done in ksmserver which is part of plasma. Also the method of KAutostart do not work cross platform and most apps simply use the X-KDE-autostart-condition, which is not affected by this MR.
In case we need to configure cross platform autostart logic the KService class needs to be rewritten anyways. But there is currently no need or plans for it. By importing this class in plasma-workspace we are more flexible in any case.
Task: https://phabricator.kde.org/T12176
The KService class will be deprecated once this is merged.