Summary:
The new Command API provides limited access to Grist Commands from within cusotm
widgets. This includes the ability to perform undo and redo, which is bound to
the same keyboard shortcut as Grist by default.
Test Plan: Browser tests.
Reviewers: jarek
Reviewed By: jarek
Subscribers: paulfitz, jarek
Differential Revision: https://phab.getgrist.com/D4050