Adds compose-chart.pl to generate DocBook/XML documents listing compose
keys, and Makefile rules to generate HTML & PDF output from them if xmlto
is present.

https://bugs.freedesktop.org/show_bug.cgi?id=19379



Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Reviewed-by: Mikhail Gusarov <dottedmag@dottedmag.net>
Reviewed-by: James Cloos <cloos@jhcloos.com>
Tested-by: Gaetan Nadon <memsize@videotron.ca>