Window order editing is practically broken

[Nami Verasche]

http://www.lemmingsforums.net/index.php?topic=2754.0

From topic:

1. When Adding, you will always get Object #0. Even when highlighting a trapdoor of a different index number, when clicking on Add it says Object #0.
2. When playing a level with more than one trapdoor, only the trapdoor with the lowest index number is where the lemming comes out.
3. Sometimes, when testing a level, it ends up with another List index out of bounds error (e.g. when I delete one of the trapdoors in a level).

[Nami Verasche]

#1 was a misunderstanding of how it works; but nonetheless, made some improvements to make it more intuitive. Waiting for feedback from user re: #2. #3 is a result of invalid data; I've made the editor less likely to save invalid data.