Compare self address in lowercase, to avoid self-replying when people
put in uppercase versions of the mail.

Reported-By: helby on Freenode

The functionality was broken since the decoding changes.
This commit also simplifies the code (in my view) to make the application logic
easier to follow.
The docs are updated accordingly (the feature was poorly documented).

As far as I am aware there should be no breaking changes (and is certainly
still in the spec of the prior documentation)

Previously the workers returned a mixture of decoded / encoded parts.
This lead to a whole bunch of issues.
This commit changes the msgviewer and the commands to assume parts to already
be decoded