This adds a "native" Wayland clipboard backend based on the
ext-data-control-v1 protocol.
This backend overcomes the limitation of the VO (Wayland core protocol)
backend, and does not require the presence of the VO window to receive clipboard
updates.
This backend also supports primary selection if supported by the compositor.
The backend runs in a separate thread, so clipboard reading
does not stall the player like the VO backend.
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