(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/WidgetAPI.ts

@@ -0,0 +1,25 @@
+/**
+ * API to manage Custom Widget state.
+ */
+export interface WidgetAPI {
+  /**
+   * Gets all options stored by the widget. Options are stored as plain JSON object.
+   */
+  getOptions(): Promise<object | null>;
+  /**
+   * Replaces all options stored by the widget.
+   */
+  setOptions(options: {[key: string]: any}): Promise<void>;
+  /**
+   * Clears all the options.
+   */
+  clearOptions(): Promise<void>;
+  /**
+   * Store single value in the Widget options object (and create it if necessary).
+   */
+  setOption(key: string, value: any): Promise<void>;
+  /**
+   * Get single value from Widget options object.
+   */
+  getOption(key: string): Promise<any>;
+}