[Frontend-GNOME] Fixed thread/memleak when reconnecting frontend (closes: #805, refs: #667)

Reconnecting to the smuxi-server was leaking all ChatView objects as
the ChatViewManager.ChatRemoved event was never raised from
ChatViewManager.Clear(). Thus StatusIconManager, NotifyManager and
IndicateManager continued to reference ChatViews objects. This leaked the
LastSeenHighlightQueue thread and probably also GTK+'s TextBuffer used in
MessageTextView.

Now we use the regular cleanup code of ChatViewManager.RemoveChat() instead.