SuperFlat customization screen is really bugged

[gus33000]

When you change the blocks, the preview of the terrain doesn't get updated

[alice-mkh]

Currently I have to copy the original implementation. So it needs an update each time Minecraft updates. Overlay system may fix that comletely, as in this case I will be able to modify the GUI without modifying its class.
It should not be too complicated to implement, but may introduce new bugs.