doc: generate server documentation from XML too

Same as WaylandClientAPI.xml we now also generate WaylandServerAPI.xml for
publication. Most of this hunk is just adding a client/ or server/ into the
xml path to keep the two separate.

The change in wayland.doxygen now causes a standard doxygen call to not
generate anything - what is generated is specified through the options
passed by make.

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