Expose the scene graph publically through a QSGEngine class

This change wraps QSGRenderContext and QSGContext in a new QSGEngine
class, and expose a public interface of QSGRenderer through a
QSGAbstractRenderer to make it usable on a standalone window or FBO.

Change-Id: I2d41187472424f5ea64650a006bcd61f2711f6b9
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>