add support for underline style in Text

QTBUG-87260 requested the ability to render a red wavy line for spell checking.  I added support for underline color because it was relatively easy; but a wavy line would probably need to be rendered with a 9-patch, so this is a new feature to implement, more than a bugfix.  QPainter can render it since 4.2.0, according to the changelog.  qpainter.cpp has static QPixmap generateWavyPixmap(qreal maxRadius, const QPen &pen); I wonder if it could be reused to generate the 9-patch.