refactor: remove isPic argument from cityMessage() notification

Have the decision of whether to show the message in the notification
pane be determined by the message type itself, rather than being
decided by the sender of the message.

git-svn-id: https://micropolis.googlecode.com/svn/trunk/micropolis-java@859 d9718cc8-9f43-0410-858b-315f434eb58c