put left margin notes in the reverse margin in LaTeX output

This change puts left margin notes in the reverse margin.  Well, LaTeX has the concept of inner and outer margins, while common/common_core.xsl seems to focus on left and right.  But anyway, this change is an improvement over the status quo (I think), and it does the right thing in one-sided layouts at least (also, in a side-by-side edition where picture is left and text is right).