logind: Add a function that detects the logind seat.

Summary:
This will allow other parts of kwin to get the current logind seat that this instance is running under

Reviewers: #kwin, graesslin

Reviewed By: #kwin, graesslin

Subscribers: graesslin, anthonyfieroni, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D9551