Thi introduces new APIs to post a `DEVICE_CHANGED` message on the
bus so the application is notifies when a device is modified. For
example, if the "defaultness" of a device was changed or any property
that can be changed at any time. Atomically changing the device
object notifying that way allow us to abtract away the internal threads.
New APIS:
- gst_message_new_device_changed
- gst_message_parse_device_changed
- gst_device_provider_device_changed
We need different export decorators for the different libs.
For now no actual change though, just rename before the release,
and add prelude headers to define the new decorator to GST_EXPORT.
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.
