egui: add redo support to `Undoer` #7687

Job	Run time
android	34s
cargo-deny aarch64-apple-darwin	40s
cargo-deny aarch64-linux-android	42s
cargo-deny i686-pc-windows-gnu	1m 4s
Check wasm32 + wasm-bindgen	2m 22s
cargo-deny i686-pc-windows-msvc	39s
cargo-deny i686-unknown-linux-gnu	52s
cargo-deny wasm32-unknown-unknown	49s
Format + check + test	4m 46s
cargo-deny x86_64-apple-darwin	39s
cargo-deny x86_64-pc-windows-gnu	39s
Check Windows	2m 28s
cargo-deny x86_64-pc-windows-msvc	40s
cargo-deny x86_64-unknown-linux-gnu	56s
cargo-deny x86_64-unknown-linux-musl	42s
cargo-deny x86_64-unknown-redox	59s
	19m 31s

Provide feedback