Introduce ext-background-effect-v1 protocol #5457

Job	Run time
clippy-check	2m 1s
format	11s
smithay-check-features	3m 4s
check-msrv	1m 20s
check-minimal	1m 17s
smallvil-check	1m 8s
smithay-tests	2m 11s
anvil-check-features	2m 49s
Documentation on Github Pages	1m 19s
WLCS: Pointer input tests	1m 8s
WLCS: Output tests	1m 7s
WLCS: Bad Buffer Test	1m 8s
WLCS: Core tests	1m 14s
	19m 57s

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