Add support for gallery command execution

Add support for the possibility to start a service that is not web based from the gallery. This way a visualizer can be started and stopped from the gallery and once DF Storyteller is closed it will terminate all the visualizers.

In order to start a service we have to know:

• What command to execute to start the service
• What command / what to do to stop the service, we could also give the SIGTERM signal to the application. And later SIGKILL if it does not stop within a few minutes.
• What to open (URL?) once service is running. Do we expect the visualizer to open itself to the user? (behind an option?)
• ...

If you have suggestions of feedback please let us know.