Difficult to intruduce the code for Custumized world type presets.

When you go to Create_New_World/More_World_Options.../World_Type:Custumized/Custumize/Presets you need to delete all the default text before you can paste (or write) the presets you want.
I suggest you to add a button that deletes all the text.
The same for superflat presets, but in superflat is a small text, so it is not so necessary.