(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-ti.ts

@@ -0,0 +1,20 @@
+/**
+ * This module was automatically generated by `ts-interface-builder`
+ */
+import * as t from "ts-interface-checker";
+// tslint:disable:object-literal-key-quotes
+
+export const WidgetAPI = t.iface([], {
+  "getOptions": t.func(t.union("object", "null")),
+  "setOptions": t.func("void", t.param("options", t.iface([], {
+    [t.indexKey]: "any",
+  }))),
+  "clearOptions": t.func("void"),
+  "setOption": t.func("void", t.param("key", "string"), t.param("value", "any")),
+  "getOption": t.func("any", t.param("key", "string")),
+});
+
+const exportedTypeSuite: t.ITypeSuite = {
+  WidgetAPI,
+};
+export default exportedTypeSuite;