Fixed edge label positioning

Fixes the label positioning for moving edges. An example can be found [here](http://algoviz.org/OpenDSA/dev/OpenDSA/AV/Development/commonTriePRO.html).

Please double check that this doesn't break anything. I removed the line
```
this.jsav.container.on("jsav-updaterelative", _labelPositionUpdate);
```
because ```_labelPositionUpdate``` is called from the edges layout function.


Btw, is there a reason why trees don't use the class ```jsavcenter```?