Refactor pop-out chat

[ctm]

Use a data representation that makes it impossible to have log be None and still think that we have docked chat.

IOW, fix the pop-up chat bug (#1550) right, rather than a quick fix. At first glance, this looks trivial to solve correctly in that we have two different fields in Table that are intrinsically tied together: log: Option<ScrollableLog> and undocked_chat: Option<chat::Parent>. AFAICT, there should always either be a log or an undocked_chat, so just slapping them into an Either fixes it for good.

My guess is that if I use an Either and then fix the compilation errors that as I'm doing so I'll have an aha moment and understand just how they could have gotten out of sync. OTOH, there's also a chance that I'll run into some spaghetti code that will take a while to navigate.

So, for now I'm labeling this high priority and easy, but I'm not going to take a look at it until I'm caught up on email.

[ctm]

FWIW, I do believe that Either (or an enum of our own) is the way to go, however, it's not trivial, because currently there is a time after we undock where we send the current chat to the child window and during that time both log and undocked_chat are Some. I doubt that it will be particularly hard to fix this, but now is not the time.