Exhibition of the week China’s Hidden CenturyBlockbuster survey of China in the 19th century, when the imperial era was coming to an end. British Museum, London, 18 May to 8 October Also showing Gwen John: Art and Life in London and Paris Introspective portraits by the Welsh Vermeer. Pallant House, Chichester, 13 May to 8 October Patrick CaulfieldEarly work by one of the most ironic and haunting British modern painters and printmakers. Josh Lilley, London, 18 May to 20 June Melati Suryodarmo This acclaimed performance artist brings her vision to…