Instead of listening on KConfigDialog's signals, this patch makes parts
listen on Okular::Settings::self()'s configChanged() signal.

Unlike KConfigDialogs, Okular::Settings::self() is unique and never
changes during the process lifetime, and therefore:
 - A part can connect during initialization and receive notifications,
   without depeding on KConfigDialog objects (which might be instanced
   or deleted by a different part).
 - The connection now survives deletion of the KConfigDialog.