Map wlroots scene API to groups / outputs / etc.

[sdilts]

The wlroots scene API seems to have stabilized, so we need to figure out how we want to map the various components that we have to nodes in the scene graph.

[sdilts]

I think the best way to structure this is to have one root scene mode in the hrt_server struct, then have one scene node per group. We can use the frame machinery that is partially implemented to handle the rest of the placement.

It looks like we are going to need to wrap the scene graph API. The things we need to do:

• Disable and enable groups of views so that we can implement group switching.
• Add and remove views from the group.
• Move views between groups