wayland: Provide information about scroll devices

wayland: Provide information about scroll devices

The Wayland protocol does not share XI2's wealth of information
about individual devices, but it does provide discriminating
information about the source for scroll events. Pass this on to
the application by creating separate slave devices for these,
and setting them as source device on the scroll events.

These devices can be discriminated by their input-source property:
wheel      - GDK_SOURCE_MOUSE
finger     - GDK_SOURCE_TOUCHPAD
continuous - GDK_SOURCE_TRACKPOINT

https://bugzilla.gnome.org/show_bug.cgi?id=767093

fix up