some essential methods added to SpriteAtlas

* GetSprite() renamed to Get()
* GetSpriteNames() renamend to GetNames()
* ObjectEnumerator, Count, IsEmpty and Contains added
* And the internal nameSet:StringSet is removed, as the childSprites:StringMap
  already contains all required informations
moved contentscaler* from kernel into kernel.scaler
subfolder sprites/ and colors/ added within graphics/
failing tests fixed
bono internal imports simplified
stop creating bono-symlinks and use full import path
make tests with new import structure fixed
first `make imports` after structure change