Add setting for charge threshold

On supported hardware, when using the device mostly on line power, this allows to stop charging the battery when it reaches a certain level as to not leave the battery fully charge for extended periods of time. It also allows to configure below which level the battery will then be charged again.

Unfortunately, UPower doesn't seem to be aware of this, so the device will just sit around the configured percentage, still claiming "charging".

The battery stop charging threshold is exposed on DBus so a hint coul be displayed in Battery monitor when a limit is configured.

Only tested on my Thinkpad so far.

BUG: 417464 FIXED-IN: 5.20