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.
When the slave device changes, the master takes the shape of the
new one, modifying its axes, this signal is more useful to catch
this situation than the n-axes property
* _gdk_device_set_associated_device() did not allow NULL device
* GdkDisplay should dispose device manager to avoid devices
trying to touch the display in finalize
* GdkDeviceManagerXI did not ref devices in id hash
* GdkDisplayX11 did not ref devices in ->input_devices
