gdk: Conflate GDK devices

gdk: Conflate GDK devices

Make GdkEvents hold a single GdkDevice. This device is closer to
the logical device conceptually, although it must be sufficient for
device checks (i.e. GdkInputSource), which makes it similar to the
physical devices.

Make the logical devices have a more accurate GdkInputSource where
needed, and conflate the event devices altogether.