Default node icon.
String, default = ''
For any node if its icon property is not defined, then default icon will be used.
The icon property is a css class for the icon span. See .nodes to see how to
use icon for nodes.
The difference between img and icon is that img
uses a div and a background image. The icon property uses a span and icon fonts, such as font-awesome.
You can define it during object creation:
Or any time after the object has been created: