Update Graph.coffee: Introduced a graph:list command

[jomeier]

This commit introduces a command which lists all graphs which were created through the fbp protocol in a runtime.

It's needed for UIs to get an overview of which graphs are available (for dynamically subgraphing, deleting, ...).

That's my proposal for this issues:
noflo/noflo#511
flowbased/fbp-protocol#25

[bergie]

In general looks good, but we should handle the protocol spec side first: flowbased/fbp-protocol#25