Make buttons for attachments allow viewing as well as saving

Define a keymap for attachment buttons to allow multiple actions.
Define 3 possible actions:
    save attachment: exactly as currently,
    view attachment: uses mailcap entry,
    view attachment with user chosen program

Keymap on a button is: s for save, v for view and o for view with
other program. Default (i.e. enter or mouse button) is save but this
is configurable in notmuch customize.

One implementation detail: the view attachment function forces all
attachments to be "displayed" using mailcap even if emacs could
display them itself. Thus, for example, text/html appears in a browser
and text/plain asks whether to save (on a standard debian setup)

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