Add a disconnect callback for BlueZ

[arthur-proglove]

This tries to fix issue #82 for BlueZ implementation only.
Other implementations (Core Bluetooth and .NET) will fail with a
NotImplementedError exception.

The disconnect event is found in the DBus PropChanged:

path: /org/bluez/hci0/dev_XX_XX_XX_XX_XX_XX, domain: org.bluez.Device1, body: {'ServicesResolved': False, 'Connected': False}

The callback will be called only on unsolicited disconnect event from
peer. An initiated disconnect from the host will not call it.

[arthur-proglove]

I am not an expert of async and await usage.
How can we call self._bus.delMatch('PropChanged') in the _properties_changed_callback routine to act the same as the disconnect routine?