Root skills

Root skills are starting point in the skill tree. They behave differently depending on exclusive_root option value set in category.json.

When exclusive_root is set to false, or is not set at all. Then all root skills are available to unlock at any time.

When exclusive_root is set to true. Then all root skills are available, but after one of them is unlocked, other root skills became locked. However they can still be later unlocked just like normal skills.