Extend Install Pipeline

[Nowi5]

With the modifications of the installer, the following improvements were made:

1. New step for own Commands, which can be set in config. So the output will be shown in the finish screen as well. Better as event triggered.
2. Loading indication, when clicking a button (esp. the last step) the loading may take some time. To avoid that user clicks multiple times on the same button, an overlay is introduced.
   
3. New configuration options to define different steps esp. for local development.