(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-ti.ts

@@ -4,11 +4,22 @@
 import * as t from "ts-interface-checker";
 // tslint:disable:object-literal-key-quotes
 
+export const RequestedInteractionOptions = t.iface([], {
+  "requiredAccess": t.opt("string"),
+  "hasCustomOptions": t.opt("boolean"),
+});
+
+export const InteractionOptions = t.iface([], {
+  "accessLevel": "string",
+});
+
 export const CustomSectionAPI = t.iface([], {
-  "createSection": t.func("void", t.param("inlineTarget", "RenderTarget")),
+  "configure": t.func("void", t.param("customOptions", "RequestedInteractionOptions")),
 });
 
 const exportedTypeSuite: t.ITypeSuite = {
+  RequestedInteractionOptions,
+  InteractionOptions,
   CustomSectionAPI,
 };
 export default exportedTypeSuite;