[lmi] Forward-declaring friend classes

[Greg Chicares]

Vadim--It surprises me that this was necessary
  
http://cvs.savannah.nongnu.org/viewcvs/lmi/lmi/illustration_document.hpp?rev=1.5
because of C++98 3.4.4/3, which gives the example
  friend struct Glob; // OK: Refers to (as yet) undeclared Glob
but if gcc-4 rejects it then we need to make this harmless change.

Did you not encounter similar problems with, say,
  class XmlNotebook [...]
  {
      friend class mvc_test;
? There are other instances of this same usage, too.