Improve formatting of PaperColumn debug info

Improve formatting of PaperColumn debug info

Paper_column::print can be very helpful in debugging spacing
information, but until now its output was not very readable.
This patch:
 - decreases font-size of debugging info so that it doesn't
   collide and overlap everywhere,
 - makes it possible to override the size of the arrow values
   with font-size property,
 - makes both red and blue arrows point in the same direction
   (previous situation was confusing)
 - centers the spring and rod values on the arrows (previously
   it was hard to discern which number goes to which arrow),
 - prints debugging info always on top,
 - makes arrows and the distance between them scale with fontsize.