[ubuntu-art] Update-notifier icon

[Lapo Calamandrei]

I checked the naimng specs, it contemplate software-update-available
and software-update-urgent:

software-update-available: The icon used when an update is available
for software installed on the computing device, through the system
software update program.

software-update-urgent: The icon used when an urgent update is
available through the system software update program.

It could be usefull to have update-mager to notify in a different way
a critical update from an optional one, in that case the bold
up-arrow/exclamation-mark icon could be used for
software-update-urgent and another subtler icon for
software-update-available.

Lapo