fb_backend_qpainter_backend: Use logind to determine if the session is active.

Summary: TTYs are only available for seat0, so when starting a framebuffer kwin on seat1, it never draws, because drawing is always suspended, because it is being treated as not active

Test Plan: The framebuffer backend draws on seat1, and /dev/fb1 and when I switch back to my Weston greeter, (when it's also using the frame buffer,) it doesn't try to draw on top of it.

Reviewers: KWin, graesslin

Reviewed By: KWin, graesslin

Subscribers: davidedmundson, rkflx, graesslin, kwin, KWin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D9574