emacs: Bind MIME part commands to "." submap

emacs: Bind MIME part commands to "." submap

Since the part commands are no longer tied to a button, but can be
applied with point anywhere within a part, bind the part commands
keymap to "." everywhere in the show buffer.  This lets you save or
view parts without having to navigate to the part button, and is
particularly useful for parts that have no button.

This removes the un-prefixed MIME part commands from the part button
keymap, but that's okay because those clashed in annoying ways with
show buffer bindings like "s" for search.  RET on part buttons is
unaffected, which is the most important part button binding.