backends: Add MetaInputDevice derivable class

This class sits between ClutterInputDevice and the backend implementations,
it will be the despositary of features we need across both backends, but
don't need to offer through Clutter's API.

As a first thing to have there, add a getter for a WacomDevice. This is
something scattered across and somewhat inconsistent (eg. different places
of the code create wacom devices for different device types). Just make it
here for all devices, so users can pick.

https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1109

src/backends/native/meta-input-device-native.c

@@ -31,7 +31,7 @@
 
 G_DEFINE_TYPE (MetaInputDeviceNative,
                meta_input_device_native,
-               CLUTTER_TYPE_INPUT_DEVICE)
+               META_TYPE_INPUT_DEVICE)
 
 enum
 {