Save schematic symbol field and unit by ordinal sheet instance.

Use the ordinal symbol reference and unit when saving schematics to
prevent file changes instead of the first instance.  The first instance
would change when sheets were moved and the hierarchy order changed.
The ordinal instance is defined as the first sheet instance when the
hierarchy sorted by page number.

eeschema/schematic.cpp

@@ -221,9 +221,7 @@ SCH_SHEET_LIST SCHEMATIC::Hierarchy() const
 
 void SCHEMATIC::RefreshHierarchy()
 {
-    wxLogDebug( wxS( "Refreshing schematic heirarchy." ) );
-
-    m_hierarchy = SCH_SHEET_LIST( m_rootSheet );
+    m_hierarchy = BuildSheetListSortedByPageNumbers();
 }