xdg-shell-v6: don't destroy role resources on unmap

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.

include/wlr/types/wlr_xdg_shell.h

@@ -106,6 +106,7 @@ struct wlr_xdg_surface {
 	uint32_t configure_next_serial;
 	struct wl_list configure_list;
 
+	// Only for toplevel
 	char *title;
 	char *app_id;
 

include/wlr/types/wlr_xdg_shell_v6.h

@@ -106,6 +106,7 @@ struct wlr_xdg_surface_v6 {
 	uint32_t configure_next_serial;
 	struct wl_list configure_list;
 
+	// Only for toplevel
 	char *title;
 	char *app_id;