Add ConsoleKit2 support for launching Wayland sessions

Summary: ConsoleKit2 as of version 1.1.1 implements the Session Controller dbus calls required by Kwin to run under Wayland. This patch first looks for the login1 service before attempting the ConsoleKit service.

Test Plan:
On a system running ConsoleKit2, logging into a text console and running:

export $(dbus-launch); export QT_QPA_PLATFORM=wayland; startplasmacompositor

Most of this patch is just shuffling code around to support both logind and CK2.

Reviewers: KWin, graesslin

Reviewed By: KWin, graesslin

Subscribers: graesslin, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D6291