Add arbitrary properties to widgets #2808

Job	Run time
cargo check (msrv) (wasm32)	14s
cargo clippy (windows-latest)	1m 13s
cargo check (aarch64-android)	26s
cargo clippy (macos-latest)	1m 9s
cargo check (msrv) (windows-latest)	2m 10s
cargo clippy (wasm32)	1m 53s
cargo clippy (ubuntu-latest)	1m 22s
cargo check (msrv) (macos-latest)	38s
cargo doc	48s
cargo check (msrv) (ubuntu-latest)	46s
formatting	38s
cargo miri (unsafe tree_arena)	35s
Prime LFS Cache	4s
cargo test (wasm32)	19s
typos	8s
cargo test (windows-latest)	4m 33s
cargo test (macos-latest)	1m 48s
cargo test (ubuntu-latest)	2m 45s
	21m 29s

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