Move QGLShaderProgram / QGLFramebufferObject and other OpenGL helper classes into QtGui on top of QGuiGLContext.

These are needed for QML 2, and we don't wish to make QtDeclarative link against QtOpenGL as that would also make QtWidgets a dependency. Instead we should move the widget-independent stuff out of QtOpenGL.

An unresolved issue is what to do about the APIs that mention QGLContext, like QGLShaderProgram and QGLFunctions. Maybe intentionally break source compatibility here by making them refer to QGuiGLContext instead, which would only require trivial fixes to existing applications.