Fix typo: Preemptivly -> Preemptively.

Fix two typos: assoziated -> associated.

Use Roboto Light, thinner lines between tiles, slightly smaller letters

For more than 4 members in a conversation, the fourth tile now contains
an ellipsis to indicate this

Notifications now have pseudotransparent icons, icons are generated in
the correct size.

Now, MUCs have icons with up to 4 tiles representing MUC members,
similar to the gmail app.

Some caveats:
As of now, no ordering is imposed on the members, we simply pick the
first (up to) 4 returned by the MucOptions object. This could be done
better, e.g. by picking more important members first (based on
affil/role), or even going so far as to order them by how recently they
sent a message in the MUC.
Also, the code generating the tiles is really messy right now (tons of
copy and paste). It seems to work though. I will clean this up soon.

Use polymorphism to avoid dynamic dispatch based on nulled parameters.
Next up: Prettier MUC icons

Normalize the interface, reduce code duplication