Adding View->Message Display and View->Text Size menus to opened message
header menu should increase ease of use.

Scenario: Random J. Hacker.
R. J. Hacker has a 17' monitor and he has 1280x1024 true color screen. He
does everything by keyboard, but he utilizes mouse because he likes to
select the paragraph he reads (there're a lot of characters on the screen,
and his eyes can get lost easily). There's a letter from an email list and
he wants to unsubscribe.  He forgot long ago what the unsubscribe method
is, but he knows a lot about e-mail lists, and he can decide how to
unsubscribe, by a row or two from the full header.

Actual Results:
His pointer is on the preview pane, thus he has to search View menu (he's
in the feeling of being angry to that crap e-mail list and doesn't think
about Alt+V, M, H).  His eyes moved to far away and gets lost again.

Expected Results:
He would click right mouse button (on the message, not just the header),
and he would search for full headers option easily.