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
