Changes to MessageComposer:
- documentation++++
- refactor the big composeMessage() a bit: Move the parts that create the inner, image and mixed body parts out in helper functions.

Now the code is a lot easier to understand, and this fixes corrupted messages when using signing and late attachments, which was introduced with HTML image support.

I've tested this with various combinations of signing, encrypting, early attachments, late attachments, plain text, HTML, embedded images etc, so there shouldn't be any regressions.

But this stuff could _really_ use some unit tests, there are over 100 combinations of possible message structures, so I couldn't test them all.

Inline OpenPGP and Chiasmus are probably broken in HTML mode (but not only since this commit).