Alternative actions when clicking on nodes

[tokee]

In the branch overlay, clicking on a node expands the sub graph by the node and its immediate neighbors. But there are multiple actions that could be taken:

• Mark only the node (level 0)
• Mark node + edges + neighbor nodes (level 1)
• Mark node + edges + neighbor nodes + their edges + their neighbors (level 2)

The last two of those could be coupled with whether the traversal should follow incoming, outgoing or both types of links.