Fix "Enter" behaviour when wxEVT_TEXT_ENTER is skipped in wxGTK

Bring wxGTK in sync with wxMSW and wxMac by activating the default
button manually if wxEVT_TEXT_ENTER handler skips the event.

src/gtk/combobox.cpp

@@ -241,6 +241,12 @@ void wxComboBox::OnChar( wxKeyEvent &event )
                     // down list upon RETURN.
                     return;
                 }
+
+                // We disable built-in default button activation when
+                // wxTE_PROCESS_ENTER is used, but we still should activate it
+                // if the event wasn't handled, so do it from here.
+                if ( ClickDefaultButtonIfPossible() )
+                    return;
             }
             break;
     }