Expose Matrix information for Item3D subclasses

Item3D has 3 matrix functions in the private implementation that would be very useful for subclasses and expensive to determine:

QMatrix4x4 QDeclarativeItem3DPrivate::localTransforms() const;
QMatrix4x4 QDeclarativeItem3DPrivate::localToWorldMatrix() const;
QMatrix4x4 QDeclarativeItem3DPrivate::worldToLocalMatrix() const;

These functions should be exposed as protected functions to make them available to subclasses.