(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/client/ui/ViewLayoutMenu.ts

@@ -45,6 +45,10 @@ export function makeViewLayoutMenu(viewModel: ViewRec, viewSection: ViewSectionR
     menuItemCmd(allCommands.dataSelectionTabOpen, 'Data selection'),
 
     menuDivider(),
+    dom.maybe((use) => use(viewSection.parentKey) === 'custom' && use(viewSection.hasCustomOptions), () =>
+      menuItemCmd(allCommands.openWidgetConfiguration, 'Open configuration',
+        testId('section-open-configuration')),
+    ),
     menuItemCmd(allCommands.deleteSection, 'Delete widget',
       dom.cls('disabled', viewModel.viewSections().peekLength <= 1 || isReadonly),
       testId('section-delete')),