fix #370 (Model runs: misaligned labels for plots on Mac)

in general, GUI stuff can end up with the wrong font size if you
don't call org.nlogo.awt.Fonts.adjustDefaultFont on everything.
so add it to PlotPanels.
use original widget as font source to fix #370

This avoids having to call org.nlogo.awt.Fonts.adjustDefaultFont
on the plot panel.
Merge pull request #390 from NetLogo/bug-370-fix

fix #370 (Model runs: misaligned labels for plots on Mac)