Otherwise the room list badges end up having to listen to `null` for a room ID, meaning they have to filter.
The notification badge count setting is the first ever setting to watch based on a room ID, so there are no compatibility concerns with this change.
Fixes https://github.com/vector-im/riot-web/issues/8936
Watchers are now managed by the SettingsStore itself through a global/default watch manager. As per the included documentation, the watch manager dispatches updates to callbacks which are redirected by the SettingsStore for consumer safety.
This implements a dream of one day being able to listen for changes in a settings to react to them, regardless of which device actually changed the setting. The use case for this kind of thing is extremely limited, but when it is needed it should be more than powerful enough.
