This moves the individual in-app notification implementation on each of
the editor panes to the editor itself, reducing redundancy and allowing
the editor to issue notifications if needed (spoiler: it will).
Show GNOME settings when adding a supported GOA service type and
it is installed, otherwise show Geary's custom password-based impl.
For GOA accounts, hide the Remove button, and enable opening the
account in GNOME Settings from the server pane.
This allows the panes themselves to manage their own headers, command
stacks, and so on. Break out the account list pane out so it's handled in
the same way as the others.
