Add the `Portal` view to Xilem (#561) #1

Job	Run time
cargo check (msrv) (windows-latest)	3m 2s
cargo clippy (windows-latest)	6m 36s
cargo check (msrv) (macos-latest)	1m 7s
cargo check (msrv) (wasm32)	2m 58s
cargo check (aarch64-android)	1m 40s
cargo clippy (macos-latest)	3m 35s
cargo check (msrv) (ubuntu-latest)	1m 41s
cargo clippy (ubuntu-latest)	4m 34s
cargo clippy (wasm32)	2m 10s
cargo doc	1m 37s
formatting	31s
Prime LFS Cache	4s
cargo test (wasm32)	49s
typos	9s
cargo test (windows-latest)	6m 27s
cargo test (macos-latest)	2m 39s
cargo test (ubuntu-latest)	5m 29s
	45m 8s

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