Blender Git Loki

D1171: Use GHash for BHead idname lookups

This patch avoids looping over bhead's linked list when looking up values by name.

Used during appaned and library loading.
Gives noticeable overall speedup loading files that used libraries. (nearly 2x on some Mango files)

Cleanup: refactor header lookups into functions

Cleanup: use POINTER_OFFSET macro