Robert Mader authored Jan 03, 2023 and David King committed Jan 04, 2023

Devices detected in `cheese_camera_device_monitor_coldplug()` may get
duplicated, as Gstreamer will emit `GST_MESSAGE_DEVICE_ADDED` for them.

While this issue could be fixed by reorganizing the initialization code,
let's go with a simple and robust solution and check if a
`CheeseCameraDevice` using the `GstDevice` is already present.