Message display tweaking.

Message display tweaking.

IMP 5 now can successfully display simple messages containing text/plain
and text/html data.  Playing around with the formatting a bit, but my
desire is to *always* show the Mime part summary for every part we can
display inline and *never* show inline parts in the attachment list.
This is important because there is no other way to
strip/download/display in a new window the parts that are shown inline.
This goes back to the way we used to display things long ago (IMP 3),
but that is probably the better way to do this. However, this has much
work that can be done to it - for now, just need to be able to display
the parts so I can continue implementing other things.