For the GDK backend We're using the GdkDeviceManager API, which maps to
Clutter's own device manager API. GDK has now moved to a per-seat device
management model, and deprecated the device manager singleton one.
In order to avoid the deprecation warnings, we'd have to implement a
model similar to the GDK one inside the generic Clutter API, but that
would also require moving all the others backend to it, which is pretty
pointless.
Instead, we can disable deprecation warnings for the
ClutterDeviceManager implementation inside the GDK backend.
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