Add PropagateOpacity for controlling the alpha of entire UI node trees #47996

Job	Run time
check-doc	8m 54s
build (windows-latest)	5m 10s
build (ubuntu-latest)	3m 21s
check-missing-examples-in-docs	30s
build (macos-latest)	2m 1s
ci	3m 6s
miri	9m 12s
run-examples-macos-metal	4m 39s
toml	1m 56s
typos	5s
check-bevy-internal-imports	4s
check-missing-features-in-docs	47s
markdownlint	1m 3s
check-compiles	4m 42s
msrv	1m 6s
build-wasm-atomics	1m 33s
build-wasm	55s
	49m 4s

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