Drop X11 Support

[danirabbit]

Mutter has dropped support for X11, so we should too

[danirabbit]

@lenemter @leolost2605 I guess there's two questions:

• How long we want to continue releasing Gala to OS 8?
• When do we want to close X11-only issues and stop accepting new ones?

[lenemter]

I think it makes sense to keep releasing Gala for OS 8 at least until we have working OS 9 builds

And I don't care about X11 anymore, so I'd be happy to close X11-only issues :)