A toothless garbageman who once wandered Hong Kong’s streets with dingy bags of ink and brushes tied to his crutches is now the subject of a major retrospective. About 300 calligraphic works by the late Tsang Tsou-choi — who is best known by his self-dubbed title, the King of Kowloon — are showing at the ArtisTree art space in a high glass tower.