notmuch.el: add functionality in notmuch search mode to add or remove tags by region

notmuch.el: add functionality in notmuch search mode to add or remove tags by region

This patch adds `-region' versions of the `notmuch-search-' commands to find
properties. It also splits up  `notmuch-add/remove-tags' into both a
`-thread' and a `-region' version. (This makes us modify
`notmuch-search-archive-thread' to use the
`notmuch-search-remove-tag-thread' function, instead of
`notmuch-search-remove-tag', for consistency.) The add/remove-tag command
called by pressing `+' or `-' will then choose accordingly, based on whether
region is active.

This version fixes a couple of errors in the first version, which led to
incorrect marking of some tags in the search view (though the actual
tagging was still correct). It's also based on current master.

I'm not sure any more if region selection is actually the correct way to
do this, or if a mutt-style message-marking method would be better. But
I didn't want a buggy incorrect version out there.