* cleaned up cgt::Texture's constructors: only two left - to create an empty texture or a non-empty one
* cgt::Texture will no longer hold a copy of the image data in local memory
* removed a lot of redundant/confusing methods
* no longer needed to call uploadTexture() even though you don't want to upload sth.
* Adapted all known code to the new interface
* Removed cgt::TextureReaderDevil

refs #613