xwayland/selection: use one X11 window per incoming transfer

This commit introduces logic for using a new X11 window for each
incoming transfer, rather than having a global window for each selection
source.

This eliminates a whole class of bugs involving multiple concurrent
incoming transfers.

For now, we retain the outgoing transfer queue, and the selection
source-specific windows to support it. Source-specific windows are no
longer used in the incoming path, and will be removed in a future PR.

Refs #1497.

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