Containing challenges recursively

[fhnaumann]

Every challenge may contain an additional challenge (optional variable at the root). This challenge may also include an additional challenge leading to a recursive chain.

The ability to "chain" resources that way creates new customization possibilities. Currently a goal with a time limit can only exist once per challenge. E.g. having two MobGoals at different times (different timer value) is not possible. This feature makes this possible by being in different challenges. Furthermore, this adds the ability to have different rules throughout the "entire challenge" (the multiple chained challenges).