Usability: 

Using Proxy Armature on groups: an Undo now doesn't force a complete re-evalution
of this system anymore, so posing works similar as for regular (non linked)
armatures - an undo step goes back to the previous pose.

Before, an undo made the entire armature go to rest pose. Bad bad :)

I need animators to carefully check - report back if there's other issues!
Tested on Gilga character from Tube Open Movie.



git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@53762 954f8c5b-7b00-dc11-b283-0030488c597c