Try to compute row height like it should be done

Try to compute row height like it should be done

Currently, our computation of row height is not completely standard:
* we ignore completely the QFontMetrics::leading() parameter

* we add arbitrarily 2 hardcoded pixels to the height.

This patch reverses these two choices, which leads to
* slightly larger spacing for MinionPro (which has a big leading).

* an additional spacing of 20% font height that depends on dpi and zoom.

Visual inspection with LibreOffice seems to imply that it disregards
the font leading but uses a interline which is 20% larger than the
font height.