gdk: Make atoms handled generically

gdk: Make atoms handled generically

This is another step towards making GdkDisplayManager backend-agnostic.

Most of the backends profit from this as their atom implementations
where generic anyway - x11 needed that to allow multiple X displays and
broadway, quartz and wayland don't have the concept of displays.

The X11 backend still did things, so I only #if 0'd some code but did
not actually update anything.