add QTextFormat::Property for table header cells

Just another instance of the usual pattern that we didn't try to write HTML the same way we read it: <table><tr><th> ... turns into <table><tr><td><span ... with bold styling to make it look like a header. We can do better, and it's not that hard. It would also benefit later attempts to style Markdown dynamically: it will be necessary to distinguish table header blocks from cell blocks.