cc-panel: make sure to always pack a widget during construction
cc-panel: make sure to always pack a widget during construction

The CC library expects the panel to have a child widget after it's
constructed, otherwise it segfaults.

https://bugzilla.gnome.org/show_bug.cgi?id=641867

Distrotech/telepathy-account-widgets,Distrotech/telepathy-account-widgets,GNOME/telepathy-account-widgets,GNOME/telepathy-account-widgets,GNOME/telepathy-account-widgets