emacs: make "+" and "-" tagging operations in notmuch-search more flexible

emacs: make "+" and "-" tagging operations in notmuch-search more flexible

Before the change, "+" and "-" tagging operations in notmuch-search
view accepted only a single tag.  The patch makes them use the
recently added `notmuch-read-tag-changes' function (renamed
`notmuch-select-tags-with-completion'), which allows to enter multiple
tags with "+" and "-" prefixes.  So after the change, "+" and "-"
bindings in notmuch-search view allow to both add and remove multiple
tags.  The only difference between "+" and "-" is the minibuffer
initial input ("+" and "-" respectively).