Syncronise environment to user systemd session

Summary: Equivalent of dbus-update-activation-environment but in runtime

Test Plan:
Changed cursor theme (with some additional fixes there)
Checked the systemd env via the property on manager1.
It had changed

Reviewers: #plasma, broulik

Reviewed By: #plasma, broulik

Subscribers: broulik, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D27054