Introduce Battery Health Mode.

When Battery Health Mode is ON, there are two new notifications: One
when the system is plugged to AC and battery reaches 80%, and other when the system is on battery and reaches 20%. These figures are recommended by http://batteryuniversity.com/learn/article/do_and_dont_battery_table to prolong battery health.

Translation strings are still only in English and Spanish.

This is a feature in response to the petition in issue #79.