Move EmptyUndoBuffer() from wxTextEntry to wxTextCtrl

It seems unlikely that we're ever going to need this function in any
other classes inheriting from wxTextEntry (i.e. wxComboBox) and both
existing implementations are for wxTextCtrl only, so define this
function there.

If really necessary, we can always lower it to wxTextEntry later, while
moving it in the other direction wouldn't be easily possible due to
compatibility concerns.

This commit is best viewed with git --color-moved option.

include/wx/textctrl.h

@@ -574,6 +574,8 @@ public:
             DiscardEdits();
     }
 
+    virtual void EmptyUndoBuffer() { }
+
 
     // styles handling
     // ---------------