Multiple Docker Engines

[bergtwvd]

Often I want to deploy a composition on one of my Docker Engines (or Swarm standalone instances if you will). From Settings I can enter the Docker Engine URL to connect to. It would be nice if this can be a drop down list of the available Engines, rather than re-entering the Engine URL everytime I want to switch Engine.

So Settings should provide functionality to define/undefine Engines, and select the active Engine.

[bergtwvd]

Something related was reported in #58 and #79.