TwoFrameLinker updateFromXMLNode() does not handle offset frames correctly

[chrisdembia]

Right now, the updateFromXMLNode() code for TwoFrameLinker will add offset frames if the location/orientation properties are nonzero, but the code will not properly set the socket connectee names to point to these offset frames. Instead, the connectee names remain as the original bodies specified in the old model.

The intent of this line was to set the connectee name to the offset frame, but it actually has no effect.

[chrisdembia]

I've created a fix, but it is intertwined with my progress on #1751. I might try to separate out the fix into a self-contained PR.