Update the handling of agenda/capture keys contextualization.
* org.el (org-contextualize-keys): Rename from
`org-contextualize-agenda-or-capture'. Fix normalization to
handle empty key replacement string.
(org-contextualize-validate-key): Rename from
`org-contexts-validate'. Allow checking against a custom
function.
* org-agenda.el (org-agenda-custom-commands-contexts): Update.
(org-agenda): Use `org-contextualize-keys'.
* org-capture.el (org-capture-templates-contexts): Ditto.
* org.texi (Templates in contexts, Setting Options): Update to
reflect changes in how contexts options are processed.