Implementation of complete IMAP two-phase "delete/expunge" behavior.

On each IMAP account, the expunge behavior can be set to expunge
messages in a folder as soon as a move or delete is performed on the
folder ("immediately"), each time the folder is polled, or only when
executed manually.

In the Message List, there is now an Expunge action in the option
menu.

In the Folder List, there is now an Expunge action in the context
menu (long-press on the folder).

For IMAP accounts, it is also possible to disable the copying of deleted messages to the
Trash folder, by setting the Trash folder to -NONE-.

Fixes Issue 536.

Separately, in WebDAV accounts, the user can now choose the
server-side equivalents of the special folders, just like for IMAP.