QHeaderView should support richtext in the labels

Reimplementing paintSection is cumbersome and not advised in the documentation, and using a style to handle CE_HeaderLabel is not convenient if different styles are supposed to be used. The memory overhead for supporting richtext is low (no need to store anything, or at most a QBitArray to avoid detection of richtext when painting), and the performance overhead is low, as few labels are visible at any given time.