Split much of BKE_pbvh.h into BKE_pbvh_api.hh.
BKE_pbvh.h is included by BKE_paint.h, which in
turn is included by large amounts of code including
RNA.
This makes it extremely difficult to change
or clean up the PBVH API, since each modification
of BKE_pbvh.h can take 20-30 minutes to compile,
even on a quad-core system with an SSD. This
commit fixes that by moving most of BKE_pbvh.h
into another file and just having the core,
external-facing interfaces in BKE_pbvh.h.
7e2659e4ab