Flow connection GUI updates preparations and RESTification (#1/#15)

- nodes and connections not are endpoints with REST properties.
  E.g. you create a not by posting to /nodes/ and remove it by
  DELETEing /nodes/:id . Same for connections.
- FlowConnection now also exposes basic configuration very
  much like FlowNodes. On creation it also takes a set of
  free form attributes configurable by the user which is used
  to indicate endpoint locations. Theoretically this would
  also be a nice fit for some of the FlowNodes attributes like
  x and y which have no real meaning to the backend.
- Epanded FlowSupervisor and Application to handle FlowConnection
  configuration updates much like FlowNodes
- Modified GUI to use the new interface. Unfortunately it turns
  out that the beforeDrop doesn't have the needed endpoint
  information available. Converted to opportunistic methods
  which have this information instead. Adding of a rollback
  method to fix this in #23 .

Left to do: Create and delete connections in response to received
configurations.