window: Support snap packages for sandboxed app IDs

For now we abuse of meta_window_get_flatpak_id not to break the APIs,
so that it's working seamlessly in gnone shell too.

Rename flatpak_id to sandboxed_app_id internally to get prepared to the future
API.

https://bugzilla.gnome.org/show_bug.cgi?id=788217

src/core/window-private.h

@@ -164,7 +164,7 @@ struct _MetaWindow
 
   char *startup_id;
   char *mutter_hints;
-  char *flatpak_id;
+  char *sandboxed_app_id;
   char *gtk_theme_variant;
   char *gtk_application_id;
   char *gtk_unique_bus_name;