Eeschema: implement new symbol library file s-expression formatter.

CHANGES: Symbol library file format has been converted to s-expressions.

Add support code for picking apart symbols at some future junction that
will allow full inheritance conversion of existing symbol libraries.  For
now, symbols arranged by unit and body style numbers are nested for round
robin testing of symbol libraries once the parser is complete.

eeschema/lib_rectangle.cpp

@@ -2,7 +2,7 @@
  * This program source code file is part of KiCad, a free EDA CAD application.
  *
  * Copyright (C) 2017 Jean-Pierre Charras, jp.charras at wanadoo.fr
- * Copyright (C) 2004-2019 KiCad Developers, see AUTHORS.txt for contributors.
+ * Copyright (C) 2004-2020 KiCad Developers, see AUTHORS.txt for contributors.
  *
  * This program is free software; you can redistribute it and/or
  * modify it under the terms of the GNU General Public License
@@ -52,10 +52,15 @@ EDA_ITEM* LIB_RECTANGLE::Clone() const
 }
 
 
-int LIB_RECTANGLE::compare( const LIB_ITEM& aOther ) const
+int LIB_RECTANGLE::compare( const LIB_ITEM& aOther, LIB_ITEM::COMPARE_FLAGS aCompareFlags ) const
 {
     wxASSERT( aOther.Type() == LIB_RECTANGLE_T );
 
+    int retv = LIB_ITEM::compare( aOther );
+
+    if( retv )
+        return retv;
+
     const LIB_RECTANGLE* tmp = ( LIB_RECTANGLE* ) &aOther;
 
     if( m_Pos.x != tmp->m_Pos.x )