Add battery level support

[whirvis]

An I/O feature representing the battery level of a device should be added when possible to the controller module. The class of the I/O feature may possibly be named Battery or InternalBattery, and its state class BatteryLevel. The state class shall have a method named getLevel(), which returns a floating point value from 0.0 for an empty battery and 1.0 for a fully charged battery. It shall also have a method called getPercentage(), which calls getLevel() and converts its value to an integer with a range from 0 to 100.

Device classes which may have a battery feature could be, but are not limited to: XboxController, Ps3Controller, Ps4Controller, Ps5Controller, Wiimote, NxLeftJoyCon, NxRightJoyCon, and NxProController.

UPDATE: The xinput module now supports the INTERNAL_BATTERY feature present in XboxController.

[whirvis]

While not fully implemented, the feature InternalBattery with state BatteryLevelZ / BatteryLevel has been added in commit 16de8ff. A later version of Ketill will fully support these features. At which point, this issue will be closed accordingly.