(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/plugin/CustomSectionAPI.ts

@@ -2,9 +2,32 @@
  * API definitions for CustomSection plugins.
  */
 
+/**
+ * Initial message sent by the CustomWidget with initial requirements.
+ */
+export interface InteractionOptionsRequest {
+  /**
+   * Required access level. If it wasn't granted already, Grist will prompt user to change the current access
+   * level.
+   */
+  requiredAccess?: string,
+  /**
+   * Instructs Grist to show additional menu options that will trigger onEditOptions callback, that Widget
+   * can use to show custom options screen.
+   */
+  hasCustomOptions?: boolean,
+}
 
-import {RenderTarget} from './RenderOptions';
+/**
+ * Widget configuration set and approved by Grist, sent as part of ready message.
+ */
+export interface InteractionOptions {
+  /**
+   * Granted access level.
+   */
+  accessLevel: string
+}
 
 export interface CustomSectionAPI {
-  createSection(inlineTarget: RenderTarget): Promise<void>;
+  configure(customOptions: InteractionOptionsRequest): Promise<void>;
 }