Enhancement: Make handles support snapping

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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 [e-mail] on 10 Jun 2015 at 2:28