Added ability to add axis labels to the plot

Added ability to add axis labels to the plot
(setXAxisLabel(QString)/setYAxisLabel(QString)).
The "padding" space around the plot area is no longer hard-coded.
The amount of padding can be set explicitly with setXXXPadding(int),
where XXX={Left, Right, Top, Bottom}.  If not explicitly set, the code
will automatically choose a good amount of padding, depending on whether
there are Axis Labels and/or Tick Mark Labels present.
Also, the placement of Tick Mark Labels is a bit smarter now; the labels
should be better centered on the corresponding tick.

CCMAIL: kstars-devel@lists.sourceforge.net

svn path=/trunk/kdeedu/libkdeedu/; revision=238085