Widget Sync - Add new widget-sync event

[joepavitt]

Description

• Adds new widget-sync event
• Updates Events Architecture & in-app helper text to detail the new functionality
• The new event ensure that multiple clients all reflect the single source of truth, without a need to refresh the page
• This functionality is locked behind the same validClient test that the msg._client data is, so that functionality is now a consistent "single source of truth" vs. "separate clients" toggle.

Notes

I will be following this up with #1294 as it'll clean the custom onSync functions out entirely, but was no need to merge it all together in the name of iteration.

Related Issue(s)

Closes #679