Rename "feet and head follow" commands

The lot of these were rather misnamed, since they're used to replace
part of the player or enemy with objects for several background effects.
https://github.com/pret/pokecrystal/pull/578#issuecomment-440996244

engine/battle_anims/helpers.asm

@@ -100,7 +100,7 @@ GetBattleAnimOAMPointer:
 	add hl, de
 	ret
 
-LoadBattleAnimObj:
+LoadBattleAnimGFX:
 	push hl
 	ld l, a
 	ld h, 0