Put Greeter below all windows

[lenemter]

Required for elementary/greeter#878
Requires elementary/greeter#886
Requires elementary/greeter#893

Makes Greeter unmovable and puts it below apps such as Initial Setup

[leolost2605]

I fully agree with the separate group and making it desktop but (going from previous experience with our wayland protocols just organically growing) I would like to have some bigger picture where we want to take the greeter before deciding on the wayland protocol (which is why I haven't added it in the initial PR yet).
Most notably what we want to do with multiple monitors:

• Greeter on primary?
• Greeter on current?
• Greeter on every monitor?

I've been thinking we might need greeter on every monitor because most notably for touchscreen we don't have a current monitor. I'm thinking of the case "connect tablet to external monitor without any accessories". Then in theory the only way to be 100% sure to be able to log in would be having it on every monitor. But I would definitely like some opinions from @elementary/ux on this

[lenemter]

@leolost2605 My idea here is to let Gala decide what to do with Greeter windows, since there's no point in letting Greeter control all of this, since it's the only user of that protocol

[danirabbit]

@leolost2605 I agree I think it would make sense to have the greeter on all displays. I think @lenemter is right though that probably the greeter app should just declare itself as a greeter and Gala should control what that means

[leolost2605]

My idea here is to let Gala decide what to do with Greeter windows

Yeah I agree I just wanted to make sure that there's nothing that needs integration between greeter and gala. I had the monitor labels in mind because they need to specify what monitor they belong to but ig the greeter should be the same for all monitors? Anything else that might need integration?

[leolost2605]

A few nits (sorry about all of the order nits but since we've done it like that for all other methods and additions I think we should keep it here too)

Also I've been thinking since we make the greeter a desktop window anyway do we still need to put it into a separate group?

[lenemter]

@leolost2605 Done

[leolost2605]

Two more comments apart from that LGTM!

[leolost2605]

LGTM! 🚀