ui: Add `widgets::Label`. #260

Job	Run time
miri	1m 28s
no_std	26s
build (ubuntu, stable, locked, true)	11m 35s
build (windows, stable, locked, false)	22m 19s
build (macos, stable, locked, false)	18m 59s
build (ubuntu, nightly, locked, false)	20m 39s
build (ubuntu, beta, locked, false)	12m 32s
build (ubuntu, stable, latest, false)	14m 7s
build (ubuntu, stable, minimal, false)	13m 5s
fuzz	15m 7s
features	24m 29s
deploy	8s
	2h 34m 54s

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