Execute scripts with the Python interpreter directly

The #! line does not work on Windows, we can configure in the default
path and override it using an environment variable. We should add the
Python (and other) interpreter to an application settings dialog very
soon.

Change-Id: Ic4db9fdfb81275117d844610555d74530b4f1801
Merge topic 'generators-python' into master

b683cac2 Execute scripts with the Python interpreter directly