ui: Move QMP commands from monitor to new ui/ui-qmp-cmds.c

ui: Move QMP commands from monitor to new ui/ui-qmp-cmds.c

This moves these commands from MAINTAINERS section "QMP" to
"Graphics".

Command add-client applies to socket character devices in addition to
display devices.  Move it anyway.  Aside: the way @protocol character
device IDs and display types is bad design.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20230109190321.1056914-10-armbru@redhat.com>