Make extended description and link text translatable.

config/schema/type_tray.schema.yml

@@ -26,10 +26,10 @@ node.type.*.third_party.type_tray:
       type: string
       label: 'Path to the image to be used as icon'
     type_description:
-      type: string
+      type: label
       label: 'Extended description'
     existing_nodes_link_text:
-      type: string
+      type: label
       label: 'Text to be used when building links to existing content'
     type_weight:
       type: integer