(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/client/ui/RightPanel.ts

@@ -337,7 +337,7 @@ export class RightPanel extends Disposable {
           // In the default url mode, allow picking a url and granting/forbidding
           // access to data.
           dom.maybe(use => use(activeSection.customDef.mode) === 'url',
-            () => dom.create(CustomSectionConfig, activeSection, this._gristDoc.app.topAppModel.api)),
+            () => dom.create(CustomSectionConfig, activeSection, this._gristDoc)),
         ];
       }),