[GREENFOOT-543] Reduce amount of code regeneration

In the Stride editor we call regenerateAndReparse whenever we need to be sure the code is up to date. At the moment, this plays safe, and regenerates the code every time. I think we can improve this to reduce the amount of regeneration (and especially, the amount of reparsing). I'm hesitant to make a big change to this just before the 3.0.3 release, but it's something to do for the release after.

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