Draw horizontally centered labels on top of their icons, not below.

Seems more widespread. This works around an issue in label pixmap
calculation where we assume the label text to be quite some pixel
higher than they really are, resulting in odd looking vertical space.
Previously that was softened with the factor 1.5, which is the wrong
approach though and lead to symbol/label overlaps in some cases.

TODO FIXME: At least in Glow mode the labelPixmap has a bunch of empty
(transparent) pixel rows on top, originating from a wrong
text height calculation. This wastes some memory, costs some render
performance, and prevents squeezing more items into roomForLabel().