(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

test/nbrowser/gristUtils.ts

@@ -1067,6 +1067,13 @@ export async function sendKeys(...keys: string[]) {
   });
 }
 
+/**
+ * Clears active input by sending HOME + SHIFT END + DELETE.
+ */
+export async function clearInput() {
+  return sendKeys(Key.HOME, Key.chord(Key.SHIFT, Key.END), Key.DELETE);
+}
+
 /**
  * Open ⋮ dropdown menu for named workspace.
  */