Another painful editing issue solved!

Adding Ipos on a Constraint just was working totally insane. Here's how
it works now in the constraint Panel:

- Button "Show" will make the IpoWindow show the Constraint Ipo
 (it didn't before, when IpoWindow was not set to correct mode)

- Button "Key" will insert a key point on current frame (and show it in
  IpoWindow)

Note that both options will create an Action too, if not existed yet.
So, without opening an IpoWindow you can already animate Constraints.
(Todo: use the auto key option to always insert on changing slider)

And; selecting a Bone will update the Constraint IpoWindow too. Note
however, that when theres multiple Constraint Ipos on a Bone, it only
shows the first... will solve that later.

source/blender/src/editconstraint.c

@@ -129,6 +129,7 @@ ListBase *get_constraint_client_channels (int forcevalid)
 			
 			return &achan->constraintChannels;
 		}
+		else return NULL;
 	}
 	/* else we return object constraints */
 	return &ob->constraintChannels;