Generalize scenarios to have a sequence of objectives instead of a single win condition

[byorgey]

The goal would be to enable challenge scenarios with a sequence of conditions that have to be fulfilled in order. Once you complete the first objective, it enables the second objective, and so on.

At the same time, I'd like to generalize a win condition to come with some text which will be displayed in a dialog when the condition is fulfilled. i.e. right now it always says "Congratulations!" but we should be able to specify that along with the win condition. This could be used to display a specific congratulatory message, but also to explain what the next objective will be. I think we can also have a goal along with each condition which can be displayed somehow (see #356).

Right now we just have a text field win which should contain the text of a swarm program. I propose generalizing this to something like the following:

objectives:
  - condition: |
      try {
        as base {has "lambda"}
      } { return false }
    goal: See if you can find a lambda.
    message: Congratulations, you found a lambda!  Now, see if you can find a tree.
  - condition: |
      try {
        as base {has "tree"}
      } { return false }
    goal: Find a tree.
    message: Good work, now you're ready for the next challenge!

Together with #355 this would give a very flexible framework for constructing multi-step challenges, which would be especially helpful for #25 .