Adds properties to the devices listed in GstDeviceMonitor by the
applemedia plugin.
These properties are:
- device.api (always set to "avf")
- avf.unique_id
- avf.model_id
- avf.manufacturer (except on iOS)
- avf.has_flash
- avf.has_torch
Everything except device.api is taken directly from the AVCaptureDevice object
provided by AVFoundation.