add missing layout exports #543

Job	Run time
Clippy	3m 10s
semver	44s
Rustfmt	14s
Tests (windows-latest)	3m 55s
Tests (macos-latest)	2m 42s
Build Wasm Examples (basic)	48s
Tests (ubuntu-latest)	2m 21s
Build Book	1m 29s
Code Coverage	9s
Build Wasm Examples (callback-example)	48s
Build Examples (basic_charts)	25s
Build Examples (3d_charts)	31s
Build Examples (images)	41s
Build Examples (financial_charts)	27s
Build Examples (kaleido)	1m 12s
Build Examples (custom_controls)	28s
Build Examples (maps)	39s
Build Examples (ndarray)	28s
Build Examples (scientific_charts)	22s
Build Examples (subplots)	24s
Build Examples (shapes)	29s
	22m 26s

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