Page Layout: Do not double-move frames

When block selecting and dragging in Page Layout editor, multiple items
with the same parent may be selected.  We do not want to move them twice
or the block will show with misaligned elements when placed.

pagelayout_editor/block.cpp

@@ -226,12 +226,26 @@ void PL_EDITOR_FRAME::Block_Move( wxDC* DC )
 
     // Move items in block
     SaveCopyInUndoList();
+
+    for( auto item: items )
+    {
+        auto parent = item->GetParent();
+
+        if( parent )
+            parent->ClearFlags( FLAG1 );
+    }
+
     for( auto item: items )
     {
         if( item->HitTest( screen->m_BlockLocate ) )
         {
             auto data_item = item->GetParent();
-            data_item->MoveToUi( data_item->GetStartPosUi() + delta );
+
+            if( data_item && !( data_item->GetFlags() & FLAG1 ) )
+            {
+                data_item->SetFlags( FLAG1 );
+                data_item->MoveToUi( data_item->GetStartPosUi() + delta );
+            }
         }
     }