Fix mailbox names not being sent to the server quoted if needed.

Since MailboxParameter inherited from StringParameter directly, it meant
that we could never send mailbox names as quoted strings. Also, the
modified-UTF-7 encoding used for mailbox names does not encode
atom-specials such as "\", so if a mailbox name contained one or more of
these, it would be sent to the mail server unquoted.

This removes the MailboxParameter class altogether, and does the
parameter conversion to/from appropriate StringParameter subclasses as
needed. Also adds unit tests for param conversion for ASCII,
atom-specials, and non-ASCII mailbox names.

Fixes issue #40
5 jobs for wip/40-backslash-in-folder-name in 14 minutes and 30 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #67123
deb-package

00:02:25

passed #67125
fedora

00:04:31

passed #67124
flatpack-package

00:09:56

passed #67122
ubuntu

00:02:13

failed #67121
fedora

00:03:25