Updater: disconnect signals when cp view is closed - Fixes #4740

When the updater cp view is created, connect to updater signals.
The updater is a singleton, and the signals are not disconnected,
then produce the errors detailed in the ticket.

This patch add a method cleanup() to SectionView class,
and execute it when the view is closed.

Signed-off-by: Gonzalo Odiard <godiard@sugarlabs.org>