Implemented run python script using saltpig

- Added simple dart API for for wamfs
- Added action to run the selected python script in the files view.
It will run the python script through saltpig. The output will be on the console.
Also it's only available in developer mode.

review: @rginda @devoncarew @gaurave

The current version of saltpig needs to be installed and configured for that to be working.
If you make this a `ContextAction`, it'll show up in context menus. Not sure if it matters for this POC.