Polling on connection and disconnection of the device

[4javier]

At the moment it looks like once the headset is connected the extension sows up only after the poll interval expires. Same thing happens on disconnection, with the indicator staying there for some time despite switch for hiding when no headset connection is enabled. I think it should listen for connection/disconnection of devices of the right class. It's useful for the user to know battery level at the exact moment he connects them.