backends: Expose InputMapping D-Bus path to determine tablet mapping

Currently, the peripheral "output" setting will be unset if Mutter is
deciding automatically the mapped output of a tablet device. In that
case, gnome-control-center will have a hard time figuring out itself
the better output to show the tablet calibration UI, unless it's hand
held by Mutter.

Add this private D-Bus interface so that gnome-control-center can look
up the output as determined by Mutter to bring the missing harmony
between both. This interface consists of a simple method to get the
mapped output for a input device node.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2605>

src/backends/meta-input-mapper-private.h

@@ -26,10 +26,13 @@
 
 #include "backends/meta-backend-types.h"
 
+#include "meta-dbus-input-mapping.h"
+
 #define META_TYPE_INPUT_MAPPER (meta_input_mapper_get_type ())
 
 G_DECLARE_FINAL_TYPE (MetaInputMapper, meta_input_mapper,
-		      META, INPUT_MAPPER, GObject)
+                      META, INPUT_MAPPER,
+                      MetaDBusInputMappingSkeleton)
 
 MetaInputMapper * meta_input_mapper_new      (void);