org: Grouptags not unique and can contain regexp

org: Grouptags not unique and can contain regexp

* lisp/org.el (org-tags-expand): Grouptags can have regular expressions as
  "sub-tags".

  The regular expressions in the group must be marked up within { }.
  Example use:

  : #+TAGS: [ Project : {P@.+} ]

  Searching for the tag Project will now list all tags also including
  regular expression matches for P@.+.  Good for example if tags for a
  certain project is tagged with a common project-identifier,
  i.e. P@2014_OrgTags.

* lisp/org.el (org-tag-alist) : New symbols for grouptags when the
  tags in the group don't have to be distinct on a heading.

  Grouptags had to previously be defined with { }.  This syntax is
  already used for exclusive tags and Grouptags need their own,
  non-exclusive syntax.  This behaviour is achieved with [ ].  Note: {
  } can still be used also for Grouptags but then only one of the
  given tags can be used on the headline at the same time.  Example:

  [ group : sub1 sub2 ]

  Grouptags also are not filtered when setting up tags.  This means
  they can exist multiple times in org-tag-alist list.  It will be
  usable if nesting of grouptags is ever to become reality.

  There is a slightly annoying side-effect when setting tags in that a
  tag which is both a part of a grouptag and a grouptag of it's own
  will get multiple key-choices in the selection-UI.

* lisp/org.el (org--setup-process-tags): Adaption for the added syntax
  for non-distinct grouptags.

* lisp/org.el (org-fast-tag-selection): Add support for the added,
  non-unique, grouptag-syntax.  Minor (if ...) to (when ...) refactor.