Implement scoped configuration change notifications

[octo]

Right now any configuration change triggers a mailSettingsSaved event, which does not contain any additional data about the change. This has the side-effect of requiring all listeners of this event to re-read the whole configuration hierarchy and do many potentially unnecessary refresh operations.

While this is okay for infrequently-modified preference items, this does have the consequence of discouraging the sort of minor preference items that a user may want to change while actively using the application.

This task is for making configuration change notifications more granular, so listeners only need to react if a portion of the configuration they actually care about has been changed.

[octo]

Implemented as of [628], though usage is still a bit crude for the sake of simplicity.