Somehow (probably during rg=off changes) we did not cache the mailbox-tree
which resulted in unneeded imap requests for information we should have
cached in the session.
With this fix we cache the mailbox-tree again and we are able to do a force
refresh.

Forced calls should be done in case of
rename/delete/add/subscribe/unsubscrive (folders.php) and
in case we have multiple imap connections and create or subscribe to a
mailbox on another client and want to sync the changes with the SM cache.
For the last reason I moved the Refresh Folder List to below the
mailbox-tree to force a refresh and added a new link on top of the
mailbox-tree to do a unseen/total refresh only.