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

admin/tests/fixtures/extension.form-schema.json

@@ -0,0 +1,68 @@
+{
+  "data": {
+    "name": "Widget",
+    "fields": [
+      {
+        "name": "name",
+        "label": "Name",
+        "type": "text",
+        "span": "left",
+        "required": true
+      },
+      {
+        "name": "color",
+        "label": "Colour",
+        "type": "text",
+        "span": "right"
+      },
+      {
+        "name": "lookup",
+        "label": "Catalogue",
+        "type": "widget",
+        "comment": "Fills in the name. Save to keep it.",
+        "widget": "acme-demo-lookup",
+        "action": "lookup",
+        "actionLabel": "Look up",
+        "fill": ["name"]
+      }
+    ],
+    "messages": {
+      "create": {
+        "other": "New widget"
+      },
+      "update": {
+        "other": "Editing widget"
+      },
+      "saved": {
+        "other": "Widget „:name” saved"
+      },
+      "deleteConfirm": {
+        "other": "Delete this widget?"
+      },
+      "deleted": {
+        "other": "Widget deleted"
+      }
+    },
+    "meta": {
+      "locale": "en"
+    },
+    "assets": {
+      "scripts": ["/admin-test/assets/acme/demo/js/lookup.js?v=abc"],
+      "styles": []
+    },
+    "redirects": {
+      "create": {
+        "redirect": "acme/demo/widgets/update/:id",
+        "redirectClose": "acme/demo/widgets"
+      },
+      "update": {
+        "redirect": "acme/demo/widgets",
+        "redirectClose": "acme/demo/widgets"
+      },
+      "default": "acme/demo/widgets"
+    }
+  },
+  "meta": {
+    "locale": "en"
+  }
+}