Add libxkbregistry to query available RMLVO

Add libxkbregistry to query available RMLVO

This library is the replacement for clients parsing evdev.xml directly.
Instead, they should use the API here so that in the future we may even
be able to swap evdev.xml for a more suitable data format.

The library parses through evdev.xml (using libxml2) and - if requested -
through evdev.extras.xml as well. The merge approach is optimised for
the default case where we have a system-installed rules XML and another file in
$XDG_CONFIG_DIR that adds a few entries.  We load the system file first, then
append any custom ones to that. It's not possible to overwrite the MLVO list
provided by the system files - if you want to do that, get the change upstream.

XML validation is handled through the DTD itself which means we only need to
check for a nonempty name, everything else the DTD validation should complain
about.

The logging system is effectively identical to xkbcommon.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>