Implement key replacement depending on the contexts.

Implement key replacement depending on the contexts.

* org.el (org-contextualize-agenda-or-capture): Handle key
replacement depending on the contexts.

* org-capture.el (org-capture-templates-contexts): Allow to
use the context as a way to replace one capture template by
another one.

* org-agenda.el (org-agenda-custom-commands-contexts): Allow
to use the context as a way to replace one agenda custom
command by another one.

* org.texi (Templates in contexts): Document the new structure
of the variables `org-agenda-custom-commands-contexts' and
`org-capture-templates-contexts'.

In the setup below, X is not a real capture template, it is just
an alias to templates A and B in .txt and .el files.  A and B are
deactivated by default in all files.

(setq org-capture-templates
      '(("X" "Nothing but an alias")
	("A" "AAAA" entry (file+headline [...]))
	("B" "BBBB" entry (file+headline [...]))))

(setq org-capture-templates-contexts
      '(("A" "A" ((not-in-file . ".*")))
	("B" "B" ((not-in-file . ".*")))
	("X" "A" ((in-file . "\\.txt")))
	("X" "B" ((in-file . "\\.el")))))

Thanks to Carsten for suggesting this "key-replacement" idea!