StIcon: add support for GIcon

Add a "gicon" property so that a GIcon can be used instead of an
icon name, while still getting icon recoloring from the theme.
Also include a compatibility wrapper in libshell until GJS has
support for interface static methods.

https://bugzilla.gnome.org/show_bug.cgi?id=622451

src/shell-app-system.c

@@ -1274,7 +1274,7 @@ shell_app_info_create_icon_texture (ShellAppInfo *info, float size)
     }
   else
     {
-      ret = st_texture_cache_load_gicon (st_texture_cache_get_default (), icon, (int)size);
+      ret = st_texture_cache_load_gicon (st_texture_cache_get_default (), NULL, icon, (int)size);
       g_object_unref (icon);
     }