device-manager: Stop duplicating ApertureCameras
Devices added in aperture_device_manager_init()
via
gst_device_monitor_get_devices()
will get duplicated as Gstreamer will
still emit GST_MESSAGE_DEVICE_ADDED
for them.
Thus ignore devices already handled.
Closes #16
Depends on !24
See also GNOME/cheese!68 (merged)