This lightens the load when including forwards for PCB_TRACK and
friends as we don't need all the transitive includes. Sometimes
you just need the enums - many users just wanted VIATYPE, or
just needed a forward.
And reduces transitive code churn when something is changed in the
header - this reduces the files affected by a change in
pcb_track.h from nearly 400 to under 200.
John Beard
113746dc81