Add output option and assignment shortcuts to action calls

[JonasKunz]

A common usecase in rules is to just assign a constant or some other value to a data key. To do this, currently a "dummy"-actions needs to be used. The goal of this issue is to add a shortcut which explicitly allows doing this assignment.

In addition, calls can "process" data. This means that the same data ey is used as out and input. To allow this, we need to add an "output" option.