Show list of deleted items in reduce layer action

Also include specifically defined padstack items.  So if a padstack
includes a unique shape on a layer, include it in the list.

We may want to reduce the sensitivity of the check in the future

Fixes https://gitlab.com/kicad/code/kicad/-/issues/21363

pcbnew/padstack.cpp

@@ -1349,6 +1349,12 @@ int PADSTACK::Compare( const PADSTACK* aLeft, const PADSTACK* aRight )
 }
 
 
+bool PADSTACK::HasExplicitDefinitionForLayer( PCB_LAYER_ID aLayer ) const
+{
+    return m_copperProps.count( aLayer ) > 0;
+}
+
+
 double PADSTACK::Similarity( const PADSTACK& aOther ) const
 {
     // TODO: Implement similarity