split logic of creation of an MessageBlock and the rendering of
html into two steps.

This is actually the first real rewrite patch. I actally don't want
to change everything at once. That's why I make some intermediate steps.

The basic idea, where I whant to go:

run otp that returns a List<MessagePart> and than render it afterwards,
like it is now done for OTP::writeBodyStr.

I started now with the writeBodyStr function, that is actually responsible
for PGP inline, to have something to test against.

The idea in the end is, that MessagePart is independend of otp.
But I didn't want to move/copy functions around, because at the moment
the whole source is changing a lot. That's why I execute the functions inside otp.

Next steps are:
make all functions that are crypto related to use CryptpBlock and than move the
crypto related function out.

I made sure, that all tests still passes.