feat(10.1-02): mount plugin widget elements and run their actions from the form

- pluginAssets loads controller scripts and stylesheets from {base}/assets/ only, once per URL
- WidgetField mounts the custom element with attributes only and posts summer-action through the typed client
- Only declared fill keys returned by the server are patched; the form turns dirty and nothing saves
- widget is a registered valueless type rendered on create and update, labelled as a group
- backend::lang.extension strings in en and pl; embedded dist rebuilt

modules/phrasebook/backend/lang/en/lang.yaml

@@ -99,6 +99,11 @@ settings:
   empty: No settings are available.
   saved: Settings saved
   back: Back to settings
+extension:
+  busy: Loading…
+  widget_failed: This control could not be loaded. Refresh the page; if it happens again, check the plugin's assets.
+  partial_failed: This section could not be loaded. Refresh the page to try again.
+  action_failed: The action could not be completed. Please try again.
 messages:
   list:
     record_count: