Don't lose reference designators when importing EasyEDA Pro/Std, LTspice, CADSTAR schematics.

Fixes https://gitlab.com/kicad/code/kicad/-/issues/19490

(cherry picked from commit 644c4f8c0a)

eeschema/sch_io/cadstar/sch_io_cadstar_archive.cpp

@@ -92,6 +92,9 @@ SCH_SHEET* SCH_IO_CADSTAR_ARCHIVE::LoadSchematicFile( const wxString&        aFi
         SCH_SCREEN* screen = new SCH_SCREEN( aSchematic );
         screen->SetFileName( aFileName );
         rootSheet->SetScreen( screen );
+
+        // Virtual root sheet UUID must be the same as the schematic file UUID.
+        const_cast<KIID&>( rootSheet->m_Uuid ) = screen->GetUuid();
     }
 
     CADSTAR_SCH_ARCHIVE_LOADER csaLoader( aFileName, m_reporter, m_progressReporter );

eeschema/sch_io/easyeda/sch_io_easyeda.cpp

@@ -640,6 +640,9 @@ SCH_SHEET* SCH_IO_EASYEDA::LoadSchematicFile( const wxString& aFileName, SCHEMAT
 
         screen->SetFileName( aFileName );
         rootSheet->SetScreen( screen );
+
+        // Virtual root sheet UUID must be the same as the schematic file UUID.
+        const_cast<KIID&>( rootSheet->m_Uuid ) = screen->GetUuid();
     }
 
     SYMBOL_LIB_TABLE* libTable = PROJECT_SCH::SchSymbolLibTable( &aSchematic->Prj() );

eeschema/sch_io/easyedapro/sch_io_easyedapro.cpp

@@ -456,6 +456,9 @@ SCH_SHEET* SCH_IO_EASYEDAPRO::LoadSchematicFile( const wxString& aFileName,
 
         screen->SetFileName( aFileName );
         rootSheet->SetScreen( screen );
+        
+        // Virtual root sheet UUID must be the same as the schematic file UUID.
+        const_cast<KIID&>( rootSheet->m_Uuid ) = screen->GetUuid();
     }
 
     SYMBOL_LIB_TABLE* libTable = PROJECT_SCH::SchSymbolLibTable( &aSchematic->Prj() );

eeschema/sch_io/ltspice/sch_io_ltspice.cpp

@@ -66,6 +66,9 @@ SCH_SHEET* SCH_IO_LTSPICE::LoadSchematicFile( const wxString& aFileName, SCHEMAT
 
         screen->SetFileName( aFileName );
         rootSheet->SetScreen( screen );
+
+        // Virtual root sheet UUID must be the same as the schematic file UUID.
+        const_cast<KIID&>( rootSheet->m_Uuid ) = screen->GetUuid();
     }
 
     SYMBOL_LIB_TABLE* libTable = PROJECT_SCH::SchSymbolLibTable( &aSchematic->Prj() );