Added new API for device "connected" status

The feature allows to know whether QBluetoothLocalDevice
is currently connected to any other device using connectedDevices() method.

Change-Id: I4d1a19ebb0b42efd1fc5d5f0f9e7816aa956fbf4
Reviewed-by: Fabian Bumberger <fbumberger@rim.com>
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>