The Gtk.Switch representing the p2p.Device's Enabled property should use
the "delayed state change" logic as described in Gtk.Switch docs, i.e.
we need to use .set_state() instead of .set_active() when we get
confirmation of the property having changed its value in the
PropertiesChanged handler. The ::active property is automatically
changed by Gtk.Switch on user input.
This way the UI gives visual feedback of when the device enable/disable
op starts and ends (or fails).
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