Option to reset the badge count every time you 'launch' a client

Created by @ matthew:matrix.org.

The idea here would be to show the incremental badge count rather than a crazy high number whose delta you have to manually calculate. This is also the behaviour WhatsApp users are used to. The badge count would be still synchronised across clients, and I guess we'd still have to keep track of where the unread messages actually are, but the badge count itself would be offset back to zero every time you launch a mobile app, or (perhaps?) start reading messages on an existing session on the web etc.