Add a combo-system for Skills

[sven-n]

In the original game, the blade knight can use 3 skills in a certain order to get a combo damage bonus (and a special effect). The combo bonus is calculated by (Strength + Agility + Energy) / 2.

It would be great if we could implement that in a more generic way, so that we can allow other characters to have some kind of combos, and also to define the combo-damage-bonus.

So what we need to define:

• How many stages need to be executed
• Which skills are allowed in which stages
• How much time is allowed between the skills
• How the damage bonus is calculated (e.g. by AttributeRelationships)
• General requirements to be able to grant the combo bonus (e.g. a completed Quest, or generally an AttributeRequirement)

[sven-n]

There is now the attribute Stats.IsSkillComboAvailable which gets set to 1 when the Dark Stone Quest is completed. The calculation of Stats.ComboBonus was added as well, for Knights.