misc tuning for release;

recovered Isabelle2012 NEWS from ae12b92c145a, except for e5420161d11d;

updated to smlnj-110.75;

tuned rendering;

ignore markup elements over empty body, which are not well-defined within markup tree and fail to work with merge_disjoint (e.g. multiple inlined positions);

tuned;

more informative error;

tuned -- recovered comments from 791157a4179a;

tuned;

tuned whitespace;