Decouple main window from server list view

There has been the idea that mailboxes should be displayed in a separate
window. Further considerations have led to slightly changed plans. The
main window should instead have a dynamic container widget which can
change the content into different views depending on what the user is
doing. Support for multiple windows could probably be added in a future
update.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>
Show mailbox view when selecting a server

We used to view the mailbox in a separate window. Now we're going toward
the single window approach, and should view the mailbox within the main
window. Replace the server list view with the mailbox view when a server
is selected.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>
Add up button to main window toolbar

We can push views to the main window but there's no way to go back. Add
a button to the toolbox that lets the user move one level up the view
chain.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>