Summary: Added boilerplate code needed to create new wigets in "Add new" menu, that are wrapped around existing custom widgets. More details can be found here: https://grist.quip.com/larhAGRKyl6Z/Custom-widgets-in-Add-Widget-menu
Test Plan: nbowser tests added to verify if item in menu exits, if widget is rendered, and right side menu has widget selection and read access selection hided.
Reviewers: georgegevoian
Reviewed By: georgegevoian
Differential Revision: https://phab.getgrist.com/D3994
