(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/common/CustomWidget.ts

@@ -37,3 +37,15 @@ export enum AccessLevel {
    */
   full = "full",
 }
+
+export function isSatisfied(current: AccessLevel, minimum: AccessLevel) {
+  function ordered(level: AccessLevel) {
+    switch(level) {
+      case AccessLevel.none: return 0;
+      case AccessLevel.read_table: return 1;
+      case AccessLevel.full: return 2;
+      default: throw new Error(`Unrecognized access level ${level}`);
+    }
+  }
+  return ordered(current) >= ordered(minimum);
+}