Better special folder detection/creation

This looks for some translatable common names for special folders like
Sent Mail, Drafts, Spam and Trash, instead of only relying on the
server's special-use or xlist extensions.  If the server doesn't report
special-use/xlist, we look for common folder names, creating them on the
server if necessary, so we always have folders necessary for tasks like
saving drafts or sent mail.

Closes: bgo #713492