Node gouping and sub-graphs

In the dialog example, it seems that there is a grouping feature for nodes. However, when moving the group, it turns out that it's just an empty node around a couple of nodes and doesn't really group them.

So I think that a real grouping feature that treats all nodes in a group as one entity would be very useful since larger graphs quickly become hard to manage. In the best case, such a group should act like a sub-graph with its own inputs and outputs that can be re-used like a function.

Start by reviewing the dialog example and the existing node movement and grouping behavior. Define the scope for treating grouped nodes as one entity, including whether reusable sub-graph inputs and outputs are required; done should be demonstrated by a group that moves as a unit and supports the agreed interface.