Terminal Titlebars Should Live in the Session Type

[marcxjo]

Eventually, we want to be able to track various elements of context associated with a given terminal graphically. Currently we track the terminal process in the titlebar, but we'll also want to be able to track e.g. session-relative Id (as we used to), profile, and potentially scenario-specific variables, like SSH-connected state or root status à la GNOME Console.

We don't want to accomplish this with the current architecture of the terminal widget. The terminal widget arguably shouldn't be aware of its outer context; this is why I removed the Id property in the first place. Instead, this should be a widget that's coupled to the terminal container when the terminal is inserted into a session.