chore(ui): Rewrite a bit `TimelineEventHandler::add` #9689

Job	Run time
Generate Crypto FFI Apple XCFramework	7m 4s
Run Complement Crypto tests / tests	10m 17s
xtask / xtask-🐧	5s
xtask / xtask-🍏	10s
Test UniFFI bindings generation	2m 34s
matrix-rust-components-swift	9m 36s
	29m 46s

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