Add wxListCtrl::IsEmpty()

Add a simple accessor for consistency with the other controls.

interface/wx/listctrl.h

@@ -950,6 +950,16 @@ public:
     long InsertItem(long index, const wxString& label,
                     int imageIndex);
 
+    /**
+        Returns true if the control doesn't currently contain any items.
+
+        Note that the control with some columns is still considered to be empty
+        if it has no rows.
+
+        @since 3.1.3
+     */
+    bool IsEmpty() const;
+
     /**
         Returns true if the control is currently in virtual report view.
      */