Add emoji insertion support to Emacs

Add emoji insertion support to Emacs

* .gitignore: Ignore the generated emoji-labels.el file.
* admin/unidata/Makefile.in (${unidir}/emoji-labels.el): Generate
the emoji-labels.el file.
(gen-clean): Delete it.

* admin/unidata/README (https): Note the source for the Unicode
file that has emoji categorisations.

* admin/unidata/emoji-test.txt: Import another Unicode file.

* doc/emacs/mule.texi (Input Methods): Document the new key bindings.

* lisp/international/emoji.el: New file.

* lisp/international/mule-cmds.el (ctl-x-map): Bind the emoji
commands.