[8SP] UI: Refactor the UI to be based on runtime graph topology changes

[ivan-cukic]

The UI currently recreates the graph when something needs to
be changed, and it requires the flowgraph to be stopped for this.

Reimplement flowgraph alteration to be message-based:

• Adding blocks should be possible while the graph is runnning
  (that is, in any state of the graph)
• Connections should be performed while graph is stopped
  or paused

Keep in mind that message-based modifications are in
a request-reply pattern which means that the 'working' state
should be communicated to the user.

Should (at least fundamentally) work with local flowgraphs as well as with remote flowgraphs