Robust user interrupts during stepping

[LinqLover]

When the user presses Cmd + . while a step is being executed, interrupt the tracer gently. At the moment, one has to complete the current step message inside the cursor manually from the interrupt debugger before abandoning it. For an elegant solution, this would require an UserInterruptException provided by the Trunk (see https://github.com/hpi-swa-lab/squeak/issues/131).