(core) Extending widget

Summary:
- Adding new option 'strictType' to widget mapping
- Refreshing mappings when widget options are changed

Test Plan: Updated

Reviewers: JakubSerafin

Reviewed By: JakubSerafin

Differential Revision: https://phab.getgrist.com/D4061

test/fixtures/sites/config/index.html

@@ -39,5 +39,10 @@
     <button onclick="mappings()">mappings</button>
     <button onclick="configure()">configure</button>
     <button onclick="clearOptions()">clearOptions</button>
+    <button onclick="clearLog()">clearLog</button>
+
+
+    <div>meta columns:</div>
+    <textarea id="log"></textarea>
   </body>
 </html>