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
8 jobs for pulse_default in 21 minutes and 13 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Preparation
passed #104898
gst indent

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manifest

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  Build
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build fedora x86_64

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  Test
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check fedora

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  Full Builds
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build android arm64 api28

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build cerbero cross win32

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build cerbero cross win64

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build cerbero fedora x86_64

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