emacs: breakout notmuch-show-advance functionality from notmuch-show-advance-and...

This patch breaks out much of the functionality of
notmuch-show-advance-and-archive into a new function:
notmuch-show-advance.  This new function does all the advancing
through a show buffer that notmuch-show-advance-and-archive did,
without all the invasive thread archiving.  The return value of
notmuch-show-advance is nil if the bottom of the thread is not
reached, and t if it is.

notmuch-show-advance-and-archive is modified to just call
notmuch-show-advance, and then call notmuch-show-archive-thread if the
return value is true.  In this way the previous functionality of
notmuch-show-advance-and-archive is preserved.

This provides a way for people to rebind the space bar to a more sane
function if they don't like the default behavior.