Yet more surface/view mapping cleanups

The same as the previous thousand changes. Call weston_surface_map() from the surface role handler when the surface satisfies mapping conditions (has content, parent has content, etc), and tie view mappedness to whether or not a view's on a layer. Mostly this involves deleting code since we have all sorts of fancy helpers these days.

I've only done very light testing on these but @rawoul can hopefully test fullscreen-shell, and I guess @mvlad / @derekf are usually the best at finding cursor issues?