Make an unskippable block

[Atlinx]

The unskippable block will prevent users from skipping any stepped statements within it. A possible implementation would be to first move stepping outside of the story director and into its own service (maybe "StepManager"). Then add the ability for BlockNodes to hold services, and finally make the UnskippableNode attach itself on startup to its Block as a service called "StepManager". This essentially lets the unskippable block intercept calls from statements in a block.

Such a design pattern could help keep code extensible without having to modify existing code. This actually is a form of abstraction similar to how interfaces work since the callers of get_service() only care about getting a service that works -- they do not care about the actual implementation of the service.