Currently the font-size combo-box shows the current font-size always on the top
if the font-combo-box becomes expanded. This allows the user to increase the
font-size with one-single click. 

Unfortunately users want to *decrease* the font-size in 50% of all cases. This
means that at least one additional click is necessary to scroll upwards.
Therefore, it would be more reasonable to center the current font-size
vertically if the combo-box becomes expanded.

I know it's just a minor issue, but I think it would greatly improve the
usability of OO.o because changing the font-size is probably one of the mostly
used features.