_gdk_device_get_axis_use() dates back to pre-sealing, when the
xi2 work began, this remaining can be gone with a public
gdk_device_get_axis_use() function already in place.
The old functions to get core pointer and devices list are gone as
well. This slice is entirely replaced internally by multidevice
handling and may just go.
The xi2 device manager now handles slaves being detached and/or
attached to a master.
gdk_device_list_slaves() has been added so it is possible to
know how slaves relate with masters. The other backends (X11 and not)
don't neeed to to anything special here since their hierarchy is
fully flat.
