Allow to add actions with custom code

[khalim19]

To achieve the ultimate flexibility, let users add custom code that acts as an action (procedure or constraint). This would be a more convenient way to specify custom code as opposed to a script/plug-in which requires additional boilerplate such as registration.

While interactively adding an action should be simple enough, it fails in case we need to specify conditions, set variables as arguments, etc.

While I don't see this to be particularly difficult to implement, executing arbitrary code written in Python can pose severe security risks.