Implement group tags

Implement group tags

* org-agenda.el (org-tags-view): Set the matcher after
preparing the agenda, as `org-tag-groups-alist-for-agenda'
might be needed.
(org-agenda-filter-make-matcher): New parameter `filter' and
`type'.  Handle group tags.
(org-agenda-filter-expand-tags): New function.
(org-agenda-filter-apply): Handle group tags.

* org.el (org-blank-before-new-entry): Tiny docstring fix.
(org-tag-alist-for-agenda): Add docstring.
(org-tag-groups-alist-for-agenda): New global variable.
(org-tag-groups-alist): New buffer-local variable.
(org-tag-alist, org-tag-persistent-alist): Handle :grouptags.
(org-group-tags): New option.
(org-toggle-group-tags): New command.
(org-mode-map): Bind `org-toggle-group-tags' to `C-c C-x q'.
(org-set-regexps-and-options-for-tags): New function, factored
out from `org-set-regexps-and-options'.
(org-set-regexps-and-options): Don't handle tags, they are now
handled separately by `org-set-regexps-and-options-for-tags'.
(org-assign-fast-keys): Handle :grouptags.
(org-mode): Use `org-set-regexps-and-options-for-tags' on top
of `org-set-regexps-and-options'.
(org-fontify-meta-lines-and-blocks-1): Fontify group tags.
(org-make-tags-matcher): Expand group tags in the matcher.
(org-tags-expand): New function.
(org-tags-completion-function): Tiny code clean up.
(org-set-current-tags-overlay): Add a docstring.
(org-fast-tag-selection): Highlight group tags.
(org-agenda-prepare-buffers): Set `org-tag-alist-for-agenda'
and `org-tag-groups-alist-for-agenda'.  Don't uniquify
`org-tag-alist-for-agenda' as we may need the grouping
information for filtering in the agenda buffer.
(org-uniquify-alist): New function.

* org-pcomplete.el (pcomplete/org-mode/file-option/tags):
Handle :grouptags.

* org-faces.el (mode-line): New face for group tags.