Add wxListCtrl::GetAlternateRowColour()

Simple accessor matching the existing setter.

Closes #17250.

interface/wx/listctrl.h

@@ -789,6 +789,14 @@ public:
      */
     void SetAlternateRowColour(const wxColour& colour);
 
+    /**
+        Get the alternative row background colour.
+
+        @since 3.1.0
+        @see SetAlternateRowColour()
+     */
+    wxColour GetAlternateRowColour() const;
+
     /**
         Determines which item (if any) is at the specified point, giving details
         in @a flags. Returns index of the item or @c wxNOT_FOUND if no item is at