[GREENFOOT-640] Tidy up the Simulation thread code

The Simulation thread on the debug VM is a bit muddled at the moment. Previously, it had code for doing Swing painting at the appropriate intervals along with various world locks. I wonder if the world locks may be removed now (since all code on the debug VM that touches anything world-related should run on the Simulation thread, I think we can treat being on the simulation thread as an implicit mutex?).

I think we can get rid of the SimulationListener mechanism eventually (the only use which will remain is one use in GreenfootScenarioViewer, I think, which probably doesn't need a whole listener/event mechanism).

I think it probably makes sense for Davin to either do this or review this, since I think he probably understands that code better than the rest of us. But this is probably best done a bit later on in the rewrite after chunks of the IDE code have been removed and it's clearer what is still needed, so not assigning it yet.

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Issue metadata

• Issue type: Task
• Priority: Medium