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.

common/base_units.cpp

@@ -2,7 +2,7 @@
  * This program source code file is part of KiCad, a free EDA CAD application.
  *
  * Copyright (C) 2012 CERN
- * Copyright (C) 1992-2018 KiCad Developers, see change_log.txt for contributors.
+ * Copyright (C) 1992-2020 KiCad Developers, see change_log.txt for contributors.
  *
  *
  * This program is free software; you can redistribute it and/or
@@ -181,7 +181,7 @@ wxString MessageTextFromValue( EDA_UNITS aUnits, double aValue, bool aUseMils, E
     else
     {
 #if defined( EESCHEMA )
-        format = wxT( "%.2f" );
+        format = wxT( "%.4f" );
 #else
         format = wxT( "%.3f" );
 #endif
@@ -526,9 +526,7 @@ std::string FormatInternalUnits( int aValue )
     double  engUnits = aValue;
     int     len;
 
-#ifndef EESCHEMA
     engUnits /= IU_PER_MM;
-#endif
 
     if( engUnits != 0.0 && fabs( engUnits ) <= 0.0001 )
     {
@@ -537,11 +535,9 @@ std::string FormatInternalUnits( int aValue )
         while( --len > 0 && buf[len] == '0' )
             buf[len] = '\0';
 
-#ifndef EESCHEMA
         if( buf[len] == '.' )
             buf[len] = '\0';
         else
-#endif
             ++len;
     }
     else