Support for proxy attributes

Namely..

```py
cmds.addAttr("nodeA", ln="proxyTranslateX", proxy="nodeB.translateX")
```

The only question is, what should it look like?

```py
nodeA["proxyTranslate", cmdx.Proxy] = nodeB["translateX"]
```

Also taking into account that we also need undo, and preferably handle it via the `DG/DagModifier`.