notmuch: Add nm_get_all_tags()

Add nm_get_all_tags(), a mechanism to return a list of all notmuch tags in the
database.
notmuch: Add TAB completion of tag: names in <vfolder-from-query>

<vfolder-from-query>'s 'Query' prompt can now use TAB completion of tag names
supplied to the 'tag:' parameter.
notmuch: Add TAB completion of tag names in <modify-labels>*

<modify-labels> and <modify-labels-then-hide> can now use TAB completion of tag
names supplied after + and -.
Merge branch 'tag_completion' of https://github.com/tsto/mutt-kz

* 'tag_completion' of https://github.com/tsto/mutt-kz:
  notmuch: Add TAB completion of tag names in <modify-labels>*
  notmuch: Add TAB completion of tag: names in <vfolder-from-query>
  notmuch: Add nm_get_all_tags()
notmuch: reuse connection in complete_all_nm_tags()

Signed-off-by: Karel Zak <kzak@redhat.com>