Add wxListCtrl::IsEmpty()

Add a simple accessor for consistency with the other controls.

include/wx/listbase.h

@@ -381,6 +381,9 @@ public:
     // Return the current number of items.
     virtual int GetItemCount() const = 0;
 
+    // Check if the control is empty, i.e. doesn't contain any items.
+    bool IsEmpty() const { return GetItemCount() == 0; }
+
     // Return the current number of columns.
     virtual int GetColumnCount() const = 0;