Camel: Remove tests/message/test3.c.

Mostly testing unused CamelMultipart API.

Remove the test so I can remove the unused API.
Remove unused camel_multipart_add_part_at().
Remove unused camel_multipart_remove_part().
Remove unused camel_multipart_remove_part_at().
CamelMultipart cleanups.

Move the public struct members into the private section.

Use a GPtrArray instead of a GList to track the individual parts.

New functions:

  camel_multipart_get_preface()
  camel_multipart_get_postface()
CamelMultipartEncrypted cleanups.

Turns out this class is trivial; just sets the internal MIME type to
"multipart/encrypted", the rest of it was unused cruft.  Debated if
this class is even worth keeping, but I'll leave it for now.
CamelMultipartSigned cleanups.

Move the public struct members into the private section.

New functions:

  camel_multipart_signed_set_content_stream()
  camel_multipart_signed_set_signature()