Add strike-through support to wxDataViewItem attributes

Implement support for this attribute only in the generic version so far,
it will hopefully be implemented for the natives ones in the future.

Also add a new toggle column to the dataview sample to check how it
works: checking the items in this column enables using this attribute
for some other ones.

Closes #18180.

interface/wx/dataview.h

@@ -703,6 +703,17 @@ public:
     */
     void SetItalic(bool set);
 
+    /**
+        Call this to indicate that the item shall be displayed in strikethrough
+        text.
+
+        Currently this attribute is only supported in the generic version of
+        wxDataViewCtrl and ignored by the native GTK+ and OS X implementations.
+
+        @since 3.1.2
+    */
+    void SetStrikethrough( bool set );
+
 
     /**
        Returns true if the colour property has been set.