Docs have also been improved, to make explicit the device
type/source accepted.

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.

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

... and remove most of the casting that used to be necessary.

Mostly missing (transfer none).

move GdkDeviceKey to gdkdevice.c because it's still used there, remove
GdkDeviceAxis entirely.

"n_events" went back from guint to gint.

Try to do inclusions in the same sequence, more or less.

* _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

The goi scanner warns about these nowadays.

https://bugzilla.gnome.org/show_bug.cgi?id=623845