power: send a notification when the main battery is reaching the EMPTY_THRESHOLD.

Specifically, the notification will appear once when the percentage reaches the LOW_THRESHOLD, and
repeatedly when the battery is at or under the EMPTY_THRESHOLD, until the system automatically hibernates
(which doesn't happen now because it's not implemented)
common: raise the EMPTY_THRESHOLD from 2 to 5.
common: add SAFE_HIBERNATE_THRESHOLD.
power: force the hibernation when the battery level reaches the SAFE_HIBERNATE_THRESHOLD.
power: send notifications on battery state changes
0.24
Merge branch 'master' into debian
debian/control: build-depend on libnotify-dev.
0.24-1