Prevent redundant broadcasts for changes of available locales

The detection of an event changing the available locales triggers the
rebuild of the menu and tooltip of the toolbar button. This action is
now...

* prevented in many unnecessary cases (e.g. the update of a disabled
  langpack).
* further delayed when many events are detected in a very short time,
  as race conditions can still be present. It should still be
  considered as a workaround.