capture backspace as msg_text_upd

GLApp/GLTextField.cpp

@@ -559,6 +559,7 @@ void GLTextField::ManageEvent(SDL_Event *evt) {
       case SDLK_BACKSPACE:
       if( m_Start!=m_Stop ) {
         DeleteSel();
+        parent->ProcessMessage(this, MSG_TEXT_UPD);
       } else {
         if( m_CursorPos>0 ) {
           m_CursorPos--;