weston-launch: Pass the DRM device to Cogl

Open the device from weston-launch, and pass it to Cogl/Clutter.
This is a preliminary cleanup for our login1 integration.
meta-weston-launch: Adapt the new evdev open callback
meta-weston-launch: Remove the "VT-switched lock"

We'll replace it by pausing the Clutter master clock.
meta-weston-launch: Pause Clutter during VT switches
Replace mutter-launch with logind integration

This uses David Herrmann's new logind sessions interface to retrieve
fds for input devices, rather than using a custom setuid helper to do
the management. This vastly simplifies the interface.

This requires systemd v210, at least.

https://bugzilla.gnome.org/show_bug.cgi?id=724604
hack to get things working for demo

have to figure out something better to put here