Enhancement: Make handles support snapping

```
It would be really good if we could define snapping for handles.

In my use case I have a box with a divider like so

---------------
|       |     |
|       |     |
---------------

and a handle to move the divider. I want it to snap to other dividers.


```

Original issue reported on code.google.com by `richard....@gmail.com` on 10 Jun 2015 at 2:28
