battery: Add support for battery level related events - #4208

The user can now specify commands to be executed when a specific battery level is reached in a certain state.
The events must be specified in the events object, and the keys in the object can be in the format

• on-[status]-[state]
• on-[status]-[capacity]
  where:
• [status] can be discharging or charging
• [state] is the name of one of the states specified in the states object
• [capacity] is the battery percentage

For example, the following configuration will send a low battery notification when the battery reaches the warning and critical states while discharging and a full battery notification when the battery reaches 100% when charging.

 "battery": {
    "states": {
        "warning": 20,
        "critical": 10
    },
    "events": {
        "on-discharging-warning": "notify-send -u normal 'Low Battery'",
        "on-discharging-critical": "notify-send -u critical 'Very Low Battery'",
        "on-charging-100": "notify-send -u normal 'Battery is full'",
        },
    ...
}