By Michael Holden and Sarah Mills LONDON (Reuters) – Kate Middleton, wife of the new heir to the throne Prince William, has grown from a commoner who captivated the future monarch to the beating heart of the British royal family, as it faces a future without the beloved queen at its helm. Born Catherine Middleton, the eldest of three children of middle-class, self-made entrepreneurs is now the Princess of Wales, a title last held by William’s late mother Diana whose celebrity power and charity work eclipsed the royals around her. Kate, by contrast, has kept a lower profile, fulfilling an incre…