Slick greeter - Add an option to add a blurred background

[Hilyxx]

• Add a blurred background to the login screen.
• Add an option in lightdm-settings to enable/disable blur effect (false by default)

[clefebvre]

This is neat, but I think it would be much better as a feature, not an option, if it applied only to the dashbox.

i.e. we'd leave the background to look crisp, but blur it only through the dashbox if that makes sense.

[clefebvre]

The shutdown dialog does that.

[Hilyxx]

That would probably be nicer. I’ll try to do that this week and see how it could be implemented.

[Hilyxx]

@clefebvre

I'm going to close this PR and open a clean one. I made quite a few changes to implement the blur effect on the dashbox and also fixed some bugs.
I need to run a few more tests and will probably submit a PR tomorrow.