emacs: Use interactive specifications for tag changes in show

This modifies all tagging operations in show to call
`notmuch-read-tag-changes' in their interactive specification to input
tag changes, rather than depending on lower-level functions to prompt
for tag changes regardless of their calling context.

Besides being more Elispy and providing a more consistent programmatic
API, this enables callers to provide two call site-specific pieces of
information: an appropriate prompt, and the set of visible tags.  The
prompt lets us differentiate * from +/-.  Providing visible tags
enables a more consistent user experience than retrieving the
(potentially different) tags from the database, and avoids a
round-trip to the CLI and database.