Textual graph representation

[ygina]

It is easier to share graphs with other people if there is a textual representation. We can display a text box on the Graph tab with a "Save" and "Reset" button. The Reset button converts the current visual graph to its textual representation in the text box. The Save button converts the textual representation to the visual graph, and should have error handling if it is an invalid format. The initial textual representation can look something like graphviz, or the JSON representation sent across the wire to the backend.