This patch extracts the rendering of tags in notmuch-show to
the notmuch-tag file.
This file introduces a `notmuch-tag-formats' variable that associates
each tag to a particular format. This variable can be customized
thanks to the work of Austin Clements. For example,
'(("unread" (propertize tag 'face '(:foreground "red")))
("flagged" (notmuch-tag-format-image tag "star.svg")))
associates a red foreground to the "unread" tag and a star picture to
the "flagged" tag.
Signed-off-by: Damien Cassou <damien.cassou@gmail.com>
notmuch-tag is extended to accept various formats of the tag changes.
In particular, user prompting for tag changes is now incorporated
here, so it is common for modes.
The tag binary and the notmuch-{before,after}-tag-hooks are only
called if tag changes is non-nil.
In all cases tag-changes is returned as a list.
Tagging functions are used in notmuch.el, notmuch-show.el, and
notmuch-message.el. There are enough common functions for tagging
that it makes sense to put them all in their own library.
No code is modified, just moved around.
