libinput/connection: Don't hardcode seat0 anymore

Summary: use a new function to get the logind, instead of assuming seat0

Test Plan: kwin still starts

Reviewers: #kwin, davidedmundson

Reviewed By: davidedmundson

Subscribers: davidedmundson, rkflx, graesslin, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D9552