This handles all the forward/back stuff for us (with the GtkButton, Escape key, Alt-←, or mouse back button.)
The editor is currently a page that's re-used. I'm not sure if it's better to use a page per account, but this works just fine.
The overview/placeholder page remains about the same, except the Add button is in an action bar.
The account editor is also about the same, except for the movement of the Back button to the header, and the Save button to the action bar. I also added a Remove button on the opposite end. I'm not sure, but debating whether we should remove the Remove buttons from the overview page since they take up space for something you probably won't do that often.

Opened empty config, added a Bonjour/IRCv3 account, toggled a few options, went back and forth to overview, and removed the account from both pages.