emacs tagging cleanup, v3

[Jameson Graef Rollins]

This is a further cleaned up and simplified version of [0].  The scope
is slightly reduced, but we still end up with a simpler user
interface, with the primary tagging functions being:

notmuch-search-tag
notmuch-show-tag

In this series only one user facing tagging function,
notmuch-search-tag-thread, is removed, since it is completely
redundant with the now more flexible notmuch-search-tag.

jamie.

[0] id:"1333845338-22960-1-git-send-email-jrollins at finestructure.net"