Track which widgets are used in dashboards

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It would be good to track which dashboards are used together. We could hook server side to an event to listen to dashboard changes or send a daily or weekly report of all dashboards including their widgets.

Problem: How to make sure we could later find out which widgets belong to a specific dashboard. We cannot send eg `Dashboard1: WidgetFooBar, ...` as other Piwik instances would report a `Dashboard1` as well. Also we cannot really generate a random number as we'd always report the same dashboard under a different random "id". Maybe we can hash it in combination with a salt etc to always get the same but we need to make sure that people won't be able to find out a salt by looking at public data etc. Also not sure as what to track. None of the current reports / visualizations would let us find out which widgets are often used together I guess. For this we need to write a custom analysis script one day possibly. Could track as events?

The issue names no files, tests, or entry points. Start by clarifying the required dashboard-widget usage data, anonymization guarantees, and whether collection should use events or periodic reports. Done means a decided, privacy-preserving way to associate widgets with dashboards across Matomo instances and analyze those associations.

Written by the indexing model from the issue text.