- Update the room display name and its avatar url when the Room Summary is updated on state events change (see `[session: updateRoomSummary: withStateEvents:]`).
- Store the riot display room name and its avatar url in the Room Summary.
- Add a new category `MXRoomSummary+Riot` to handle avatar display.
TODO: Increase file store version in `MXFileStore.h`.
Changes:
- If the room has only two members, use the avatar of the second member even if this member is invited.
- The plain solid circle (without initial) is used only for an "empty room" without display name (We name "empty room" a room in which the current user is the only active member).
