Previously there was no way to see if autopack was enabled. Now the external
data menu has 3 entries instead of 2:
* Automatically Pack Into .blend (with checkbox to indicate autopack on/off)
* Pack All Into .blend
* Unpack All Into Files
Fixes T37608, includes modifications by Brecht from the original patch.
Reviewed By: brecht
Differential Revision: http://developer.blender.org/D118