Sets the amount of spacing in Pango unit between the lines of the layout.
When placing lines with spacing, Pango arranges things so that
line2.top = line1.bottom + spacing
Note: Since 1.44, Pango defaults to using the line height (as determined by the font) for placing lines. The
spacing set with this
function is only taken into account when the line-height factor is set to zero with
the amount of spacing