(core) Widget options api

Summary:
Adding configuration options for CustomWidgets.

Custom widgets can now store options (in JSON) in viewSection metadata.

Changes in grist-plugin-api:
- Adding onOptions handler, that will be invoked when the widget is ready and when the configuration is changed
- Adding WidgetAPI - new API to read and save a configuration for widget.

Changes in Grist:
- Rewriting CustomView code, and extracting code that is responsible for showing the iframe and registering Rpc.
- Adding Open Configuration button to Widget section in the Creator panel and in the section menu.
- Custom Widgets can implement "configure" method, to show configuration screen when requested.

Test Plan: Browser tests.

Reviewers: paulfitz, dsagal

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D3185

app/server/lib/WidgetRepository.ts

@@ -68,6 +68,10 @@ export class WidgetRepositoryImpl implements IWidgetRepository {
 class CachedWidgetRepository extends WidgetRepositoryImpl {
   private _cache = new LRUCache<1, ICustomWidget[]>({maxAge : 1000 * 60 /* minute */ * 2});
   public async getWidgets() {
+    // Don't cache for localhost
+    if (super._staticUrl && super._staticUrl.startsWith("http://localhost")) {
+      this._cache.reset();
+    }
     if (this._cache.has(1)) {
       log.debug("WidgetRepository: Widget list taken from the cache.");
       return this._cache.get(1)!;