Use gala instead of the greeter compositor

[leolost2605]

Fixes #871

[lenemter]

@leolost2605 Do you mind merging this?

[leolost2605]

I would be fine with merging though there are some things I'm unsure about:

• I haven't tested the installer path at all
• Is there something that we would lose support for when the greeter runs on wayland and not x?
• There are still some minor issues (e.g. wingpanel black bar, and now with making the greeter a desktop window I noticed you have to click it once before it is focused) but they should be easy fixes

Those questions are ofc only relevant if we want to make another greeter release for OS8, do we?

[lenemter]

@leolost2605 The installer path is untouched in this branch, and releasing Greeter to OS 8 is problematic, since Greeter already runs on Wayland since #752

AFAIK installer session runs greeter-compositor and launches Installer via autostart file, without the need for Greeter, so I guess it's fine