Describe the solution you'd like
Nearly all property changes on users do not trigger events which makes detecting them rather hard to detect. The framework should provide a way for plugins to detect changes and act upon them precisely - instead of crawling over all clients each time. This would be similar to the channeledited event which already offers a map of changed properties.

The implementation approach could also feature some sort of change handler that could be registered on a property.
Lastly, this could also be used for channels or basically all data holders in general.

Describe why you would like to see this implemented
A reduction in boilerplate code would be the immediate result as all plugins that work with changing client properties will no longer have to crawl over all/watched channels after a cache-update.

Additional context
Feature requested by @genius42 with regard to being able to detect changes in assigned server groups.