Add arbitrary properties to widgets (#873) #2854

Job	Run time
Prime LFS Cache	9s
cargo clippy (windows-latest)	7m 51s
cargo check (msrv) (wasm32)	19s
cargo clippy (macos-latest)	2m 6s
cargo check (msrv) (windows-latest)	6m 53s
cargo check (aarch64-android)	38s
cargo clippy (ubuntu-latest)	3m 13s
cargo check (msrv) (macos-latest)	1m 12s
cargo check (msrv) (ubuntu-latest)	44s
cargo clippy (wasm32)	1m 51s
cargo doc	1m 48s
formatting	39s
cargo miri (unsafe tree_arena)	41s
cargo test (wasm32)	26s
typos	5s
cargo test (windows-latest)	4m 11s
cargo test (macos-latest)	2m 17s
cargo test (ubuntu-latest)	4m 9s
	39m 12s

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