This method seems completely unnecessary, the base
wxTextEntry::DoSetValue(), which delegates to Remove() and WriteText(),
seems to work just as well and avoids code duplication between this
method and wxTextCtrl::WriteText().
Notice that gtk_text_buffer_set_text() is just a trivial wrapper around
gtk_text_buffer_delete() and gtk_text_buffer_insert() anyhow, so there
is no efficiency loss in not using it neither.