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Add methods to add/remove the providers that should be hidden by this
provider. Also make a method to get a list of hidden providers.

This makes it possible to have multiple systems monitor the same devices
and remove duplicates.

Add a property to see all devices, even duplicate ones from hidden
providers.

Protect against wrong arguments.
Clean up the header file indentation.

Also add GstDevice and GstDeviceMonitorFactory
And add code to the registry to save them

https://bugzilla.gnome.org/show_bug.cgi?id=678402