Low battery notification

Created by: Lekensteyn

(some code paths are related to #27 (closed))

Currently the battery status in the UI is updated unconditionally. If a the battery status is not changed, it is still overwritten.

If notifications for low battery levels are added, it is probably not a bad idea to compare the values before setting them (relying on notifications preventing duplication is not enough, if the battery level keeps updated, the user would get annoyed by many notifications).