Proxy Objects revisited!

The first incarnation assumed that proxies were local objects per
definition. Unfortunately that makes it impossible to - for example -
reference-link an entire Scene with proxies, to be used as a special
character set.

This commit makes the proxy implementation also a bit more clear.

Related work: the scene-sets were not executed fully or correctly for
the dependency graph. That happens now (in 3d view) as well.

source/blender/src/editconstraint.c

@@ -620,7 +620,7 @@ void add_constraint(int only_IK)
 		if(pchanact==NULL) return;
 	
 		/* check protection */
-		if(OB_IS_PROXY(ob) && (pchanact->bone->layer & arm->layer_protected)) {
+		if(ob->proxy && (pchanact->bone->layer & arm->layer_protected)) {
 			error("Bone is Proxy protected");
 			return;
 		}