removed grass plane, to save some space, added level 19 medium
exchanged level 13 and 16 to make sure that no upgrade is used before it is introduced in a corresponding tutorial
added resize gate tutorial, changed property of resize gate upgrade to avoid conflict with scaling of the model
removed background.jpg, instead used TextureRegion of the spacesphere texture, removed asteroid normal, because it is not possible to use with .g3db -> apk size reduce from 14.7 to 11.0 mb