Added place holders for the Billboard modifier

  - UI place holder in bl_ui/properties_data_modifier.py
  - Modifier data structure in makesdna
  - Modifier RNA structure in makesrna
  - Icon drawing code in editors/space_outliner
  - Modifier init, apply functions and declaration

A unique icon has not yet been included, at present MOD_ICON_WIREFRAME
is being used for the Billboard modifier.

File Changes:
  modified:   release/scripts/startup/bl_ui/properties_data_modifier.py
  modified:   source/blender/editors/space_outliner/outliner_draw.c
  modified:   source/blender/makesdna/DNA_modifier_types.h
  modified:   source/blender/makesrna/RNA_access.h
  modified:   source/blender/makesrna/intern/rna_modifier.c
  modified:   source/blender/modifiers/MOD_modifiertypes.h
  new file:   source/blender/modifiers/intern/MOD_billboard.c
  modified:   source/blender/modifiers/intern/MOD_util.c
Added modifier place holder documentation