Add wxTextCtrl::EmptyUndoBuffer()

This is implemented for multi-line controls under macOS only currently.

interface/wx/textentry.h

@@ -165,6 +165,16 @@ public:
     */
     virtual bool CanUndo() const;
 
+    /**
+        Delete the undo history.
+
+        Currently only implemented under macOS and only for multiline text
+        controls, does nothing in the other ports.
+
+        @since 3.1.6
+    */
+    virtual void EmptyUndoBuffer();
+
     /**
         Sets the new text control value.