RFC: Use QApplication::font with respective widget class

Ensures it picks the right font, for example QHeaderView for tab bar headers.

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Didn't really do much testing beyond the tab bar. There could be funky side-effects.

Also doesn't influence the size of the header, unfortunately, despite already having in initStyleOption

const QFont font = qApp->font("QHeaderView");
        opt->fontMetrics = QFontMetrics(font);

@ahiemstra @cblack @mart