Returns the `/dev/input/event*` path of this device.
For `GdkDevice`s that possibly coalesce multiple hardware devices (eg. mouse, keyboard, touch,...), this function will return null.
This is most notably implemented for devices of type gdk_source_pen, gdk_source_tablet_pad.
this |
a `GdkDevice` |
the `/dev/input/event*` path of this device |