Don't delete default intersheetrefs field from global labels.

Fixes https://gitlab.com/kicad/code/kicad/issues/14493

eeschema/sch_label.h

@@ -100,6 +100,11 @@ public:
         m_fields = aFields;     // vector copying, length is changed possibly
     }
 
+    void AddFields( const std::vector<SCH_FIELD>& aFields )
+    {
+        m_fields.insert( m_fields.end(), aFields.begin(), aFields.end() );
+    }
+
     /**
      * Increment the label text, if it ends with a number.
      *

eeschema/tools/sch_edit_tool.cpp

@@ -2165,7 +2165,7 @@ int SCH_EDIT_TOOL::ChangeTextType( const TOOL_EVENT& aEvent )
             SCH_LABEL_BASE* new_label = dynamic_cast<SCH_LABEL_BASE*>( newtext );
 
             if( label && new_label )
-                new_label->SetFields( label->GetFields() );
+                new_label->AddFields( label->GetFields() );
 
             if( selected )
                 m_toolMgr->RunAction( EE_ACTIONS::removeItemFromSel, true, item );