#40 Difficulty levels

[Robert Kety]

As part of the combat system, a difficulty level needs to be implemented. All combat calculations should include a probability modifier linked to a global variable. The global variable will have a default setting, but should be linked to the Difficulty slider on the Options GUI.

This ticket also requires the implementation of a skill check function that will also require dynamic probability adjustment based on the Difficulty global variable. Skill check functions are closely related to combat system capabilities, with a few exceptions (e.g., climbing), which is why this ticket is assigned to the Combat System Milestone.