We now have a proper MASTER/SLAVE input device split, where
the masters are virtual core input devices and we add fake hw
slave devices for the system pointer and real slave devices for
wintab devices.
We also set the proper source_device on the events so you can
tell which device sent it and properly decode the axis info.
_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.
There are sure regressions but basic stuff seems to be working
again after all the API breakage done with comments like
"Win32 and Quartz need to be ported still."
