power: Use distinct icons when showing battery notifications

UPower specifies the battery-caution-symbolic icon for all levels that gnome-settings-daemon considers notification-worthy. These notifications look too much alike, which may desensitize the user in the last actually urgent states.

Decouple the icons names from those specified by UPower for our own notifications, so that design changes don't necessarily involve changes in system components. And use the additional new icons added to the Adwaita icon theme to convey a higher sense of urgency to each of the notifications spun until shutdown/suspend/hibernate.

Closes: #731 (closed)