Supported a "cooked" mode for attachment filenames, which gets rid of any unacceptable characters. Add "raw_filename" to get the filename as specified in the part. Update tests.
Add more of the Message class (and various helpers), which are to replace all the `mu-msg-*` code. Add more tests.
Basically, move/rename lib/mu-message* to lib/mu-*. Add the beginnings of a Message class.
