[PATCH 0 of 5] Improve mutt's label/tag/keyword handling
Mutt has long had a loose idea of "labels", since before any other
approach to mail tagging was standardized and before most others were
implemented. Labels were intentionally rather freeform and support was
intentionally minimal, since mutt users like flexibility and power.
However in the meantime support for message tagging and keywording has
grown in many other MUAs, and the implementations vary. Mozilla has the
X-Mozilla-Keys: header. Dovecot has the X-Keywords: header. IMAP has
its own protocol support for Keywords that does not specify back-end
storage. RFC2822 added a Keywords: header. Mutt and a few other tools
use X-Label:.
This patch series improves mutt's support of labels / tags / keywords.
It works to support other implementations' header formats, and unifies
them through a single management mechanism in mutt. It also extends
mutt's capabilities to what modern "tagging" system users expect by
adding a label editor, a sort-by-label capability, and label completion
in the label editor and the search/limit pattern editor.