Api tweak

as far as I can see, this _could_ be pulled in master without breaking stuff... however, the API still needs tweaking  ( @LightningPig333 please look at the comments) I am unsure if some methods are necessary, and maybe adding a few safety checks when registering dusts would be better (although I'm trying to avoid cycling through the whole array to do so). Also, I am unsure of how to deal with the placing of the decorative dusts: will a (modified) copy-paste of the onItemUse from the regular dusts be enough or will it need to extend IDust ?