Implement the greeter

[leolost2605]

This allows running gala as the greeter compositor. It does that while preparing for having an in session lock screen.

It introduces:

• an environment variable that controls the session type
• a way to specify shell clients for a certain session type only
• a greeter layer that contains the greeter and greeter shell window groups as well as the blurred background which is always active when the session is a greeter session and will serve as the lock screen in the desktop session (the second part is more long term and not yet implemented)

This is already enough to run gala as the greeter compositor without regressions. The long term goal is that the greeter layer with the greeter and greeter shell window groups will also be used as a lock screen without switching ttys. This allows for a bunch of things like e.g. smoother animations, lock screen notifications, media controls etc.

Any feedback is appreciated especially regarding architecture and how we should animate the activation/deactivation of the greeter layer. This animation will in the future be used for the lock screen but right now it would also be used when logging in to animate out the greeter (aka to transition from the blurred background in the greeter to the normal session).

Goes with elementary/greeter#878

[leolost2605]

This is pretty much ready. It took so long because I wanted to already prepare for an in session lock screen and I wasn't sure what the best way would be. So any opinions and suggestions regarding architecture are much appreciated. The animation for the deactivation of the greeter can probably be left to a follow up because without it the behavior is exactly the same as with the greeter compositor.
Note that the in session lock screen probably requires a pretty big rewrite of the greeter because (I think? but I didn't investigate much further) we can't use lightdm from within the session.

I've been using this together with the osk branch for a while now on my starlite and it's working great with no need for any keyboard at all.