emacs: show: w3m/invisibility workaround

There is a bug in the current notmuch code with w3m and invisible
parts. w3m sets a keymap, and if we have a hidden [text/html] point
at the start of the following line still gets this w3m keymap which
causes some strange effects. For example, RET gives an error "No URL
at Point" rather than hiding the message, <down> goes to the next link
rather than just down a line.

These keybinding are also inconvenient when the text/html part is
displayed so we ask w3m not to install a keymap.

This is only likely to be a problem for emacs 23 as shr is preferred
as html renderer on emacs 24 (although the user can set the renderer
to w3m even on emacs 24).

This solution was suggested by Tomi Ollila <tomi.ollila@iki.fi>