device-provider: Allow notifying application of device changes

device-provider: Allow notifying application of device changes

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