Collapse a node's children after double-clicking

[prrao87]

Related to #6.

When double-clicking a node, we expand the graph by showing all its children nodes and associated edges. However, there's no way to go back, once the children are displayed. Along with the "hide" node, we should look at providing a "hide children" option to allow the user to prevent the graph from exploding in size.

[mewim]

Added this action to side bar.

[prrao87]

Makes sense 👌🏽.

Can we change it from "Unexpand neighbors" to "Collapse neighbors" instead?