FR: ability to store custom data for room's account_data

I would like to attach some custom data to the room's account_data. It would be nice if there was a way to extend (subclass) MXRoomAccountData and override the event handler to populate the additional information in the object whenever it updates.

I might propose a patch for this if I get it working.

Start by locating MXRoomAccountData and its event-handler path in the SDK. Determine how room account_data updates populate the object and how subclassing or another extension point could preserve custom data. Done means a documented, working way to attach and update custom room account data; no test or file is named in the issue.

Written by the indexing model from the issue text.