Print: add method to convert profile to grayscale

Add the method PrintLayout::convertPixmapToGrayscale()
to convert a rendered profile pixmap to grayscale.
It will probably be faster to have
ProfileGraphicsView render in grayscale under certain
conditions (and use a specific color table) instead -
e.g.:

ProfileGraphicsView::setPrintMode(bool printMode,
	bool useGrayScale);

Signed-off-by: Lubomir I. Ivanov <neolit123@gmail.com>