Both RFC mailbox address names and mailboxes/local-names may by RFC 2047
be Quoted-Printable or Base64 encoded. This patch ensures these parts are
correctly decoded when parsing a RFC 822 message, so that they are
displayed to the user in human-readable form.

Part 2 of Mailsploit mitigation.

* src/engine/rfc822/rfc822-message.vala (Message): Since GMime.Message's
  convenience properties for accessing header values such as senders,
  recipients, etc. in string form are presented as human-readable, not
  RFC822 compliant strings, we can't re-parse them for use by this class
  when it is being constructed from a GMime-based source. Instead,
  iterate over all headers to get the raw values and parse those we are
  interested in instead. Add unit tests.

* src/engine/rfc822/rfc822-mailbox-address.vala (BaseObject): Add gmime
  constructor so we can handle construction and decoding from a GMime
  InternetAddressMailbox object in a consistent way. Ensure both names
  are decoded correctly, and mailboxes are decoded at all, from both
  GMime and IMAP sources. If a GMime source's address has no @-symbol,
  try to decode the whole thing first it in case the whole address is
  encoded. Add unit tests.

* src/engine/rfc822/rfc822-mailbox-addresses.vala (MailboxAddresses):
  Add append method and handle group addresses here instead of in Message
  to simplify updated Message implementation.

* src/engine/rfc822/rfc822-message-data.vala (MessageData): Add append
  method to simplify updated Message implementation.