The motivation for this is:
- `get_popup` and `get_toplevel` allocate role-specific resources.
- On the first non-null commit, the surface gets mapped.
- On a null commit, the surface gets unmapped. It can be mapped
again with a non-null commit.
- When the role object (xdg-toplevel or xdg-popup) is
destroyed, the surface is unmapped and role-specific resources
are destroyed. The client can call `get_popup` or `get_toplevel`
again on that surface.
- When the xdg-surface object is destroyed, the surface is
unmapped, role-specific resources are destroyed and the surface
itself is destroyed.