Add `Ui::is_sizing_pass` for better size estimation of `Area`s, and menus in particular #11402

Job	Run time
Format + check + test	5m 53s
Check wasm32 + wasm-bindgen	2m 16s
cargo-deny aarch64-apple-darwin	45s
cargo-deny aarch64-linux-android	54s
android	34s
cargo-deny i686-pc-windows-gnu	48s
cargo-deny i686-pc-windows-msvc	42s
cargo-deny i686-unknown-linux-gnu	53s
Check Windows	2m 25s
cargo-deny wasm32-unknown-unknown	40s
cargo-deny x86_64-apple-darwin	46s
cargo-deny x86_64-pc-windows-gnu	50s
cargo-deny x86_64-pc-windows-msvc	41s
cargo-deny x86_64-unknown-linux-gnu	38s
cargo-deny x86_64-unknown-linux-musl	41s
cargo-deny x86_64-unknown-redox	40s
	20m 6s

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