[bluez-qt] Support org.bluez.Agent.DisplayPasskey pairing functionality.

This is required to support pairing requests to/from a device with
KeyboardOnly agent capability. The 'entered' parameter has been removed
from BluetoothAgentAdaptor::DisplayPasskey() as it is not actually
provided by BlueZ 4.101 and so cannot be used with this version.
Merge pull request #14 from blammit/display-passkey

[bluez-qt] Support org.bluez.Agent.DisplayPasskey pairing functionality.