When packing the custom widget, unpack the label and viceversa
When packing the custom widget, unpack the label and viceversa

We also need to add a reference to the label and release it in the
dispose, which has been added too.

Fixes: NB#143625 (Timed custom banner interferes with timed normal
banners)

community-ssu/hildon,archlinuxarm-n900/libhildon,community-ssu/hildon,archlinuxarm-n900/libhildon,community-ssu/hildon,archlinuxarm-n900/libhildon