[MlMt] Show HTML source doing The Wrong Thing

[Seebs]

It is pretty much a safe bet that, no matter what I *do* want, "show me 
the HTML that is used internally by Mailmate to render the plain text 
part of this message, but which does not actually occur in the message 
in any way" is not it.

I sometimes get mail where the sender has very carefully added an EMPTY 
plain text alternative. Which I get by default because I tend to prefer 
plain text to begin with. If I accidentally select "Show HTML Source" 
instead of "Message Body Parts -> HTML", then... Whoops! I get something 
that looks like HTML, so I might mistakenly believe there is actually 
some relationship between it and the HTML message, but which is not 
actually part of the message.

-s