Implement EVT_CHAR generation for wxDataViewCtrl under Mac

This requires manually sending the original key event back to the native
control because NSOutlineView doesn't implement the NSTextInputClient
protocol and so doesn't allow handling the keys in the usual way.

include/wx/osx/dataview.h

@@ -285,6 +285,7 @@ protected:
  // event handling
   void OnSize(wxSizeEvent &event);
   void OnMouse(wxMouseEvent &event);
+  void OnChar(wxKeyEvent &event);
 
 private:
  // initializing of local variables: