code: func (v *LevelBar) AddOffsetValue(name string, value float64) {
  	cstr := C.CString(name)
  	defer C.free(unsafe.Pointer(cstr))
  	C.gtk_level_bar_add_offset_value(v.native(), (*C.gchar)(cstr), C.gdouble(value))
  }
docs: // AddOffsetValue() is a wrapper around gtk_level_bar_add_offset_value().