Removes explicit setting of sizes in ConversationView and StatusView,
allowing them to be resized appropriately. Also rearranges StatusView
somewhat, so that it's more friendly to smaller sizes. (The avatar is
moved from being beside the status menu field to being beside the
nickname textbox.)
All chats now stay in a single window, which removes and switches its
current ConversationView with the appropriate one using
Conversation::GetView().
This is a stepping-stone to merging the ChatWindow into the MainWindow.