when drawing KPlotObject::LABEL items, the position of the label is now

when drawing KPlotObject::LABEL items, the position of the label is now
centered on the data point's X-coordinate, and slightly below the data
point in Y.  This is useful for (and in fact assumes that one is) plotting
the labels next to KPlotObject::POINT items that have the same
coordinates.  (This change was implemented in order to add planet labels
to the solar system plotter in KStars).

TODO: make the vertical offset adjustable.  Should be zero by default.
(have to wait for after 3.2)

svn path=/trunk/kdeedu/libkdeedu/; revision=251365