Use boost::signal in SmElementsControl

Change-Id: Icb19466db6720350e105b75cb1c0c858f39403ef
Reviewed-on: https://gerrit.libreoffice.org/12509
Reviewed-by: Marcos Paulo de Souza <marcos.souza.org@gmail.com>
Tested-by: Marcos Paulo de Souza <marcos.souza.org@gmail.com>