It required rewrite of TextureManager. Now its API became much simpler
there are just 3(+1) static functions. One to obtain image and two
to bind texture.

Most important changes:

* Texture IDs are cached by Qt so there no point cache them twice
* Texture manager do not own GL context any more. Instead it is
passed as parameter of QGLWidget* type.
* Texture class is not used anywhere but is still here.
* All Q_ASSERTS on QGLContext are gone
* Planet and deep-skyes store their images as QImage not as
textures. Also they are loaded at construction time.

Unfortunately this is rather large changeset but there is no
obvious way to implement these changes incrementally.