Fix multiple topic-updating issues.

First, IrcChannel setTopic messages were not previously parsed,
meaning that topic changes never showed up in Quasseldroid.  This
patch adds support for such messages.  Second, topic changes never
notified observers, causing the GUI to not be updated until the
selected buffer was changed.  This patch causes topic changes
(and, in the case of networks) network name changes, network server
changes, changes in the number of users, and changes in the lag
time to notify observers so the GUI is updated immediately.