Previously we were creating a notification state whenever we needed one, which was leading to hundreds of listeners even on a small account. To ease the burden, and reduce the load of having to wake so many listeners, we now record a single listener for each tag ID and room combination.
This commit also introduces a number of utilities to make future notification work a bit of an easier transition, such as the `isX` and `hasX` getters on the new NotificationState abstract class. Similarly, "snapshots" have been added to reduce code duplication between different kinds of states checking for updates.
The ListNotificationState is now heavily tied into the store which offers it to help reuse the cache of room notification states.
Fixes https://github.com/vector-im/riot-web/issues/14370
