Right now, we have a field in appinfo.json named surface_role. This is wrong, and it is not matching the actual types in agl-shell and agl-shell-desktop (that we do not implement anyway).

Move to have two different types:

• surface_role matching agl-shell-desktop protocol values. Doing nothing at least for now as we do not implement agl-shell-desktop.
• surface_type with just three values: none, background and panel (that behaves as current surface_role value).

For implementing backwards compatibility, we can leave surface_role as is, and eventually ignore it, and define new fields agl_surface_type and agl_surface_role (and rename panel_type to agl_panel_type).