Update all gtk crates to 18.x #5750

Job	Run time
druid doctests beta	1m 31s
druid doctests	3m 23s
mdbook build	2m 1s
Docs (macOS-latest)	3m 37s
Docs (windows-2019)	3m 34s
cargo clippy+test beta	4m 57s
cargo fmt	14s
cargo clippy+test (macOS-latest)	1m 26s
Docs (ubuntu-latest)	5m 24s
cargo clippy+test (windows-2019)	1m 26s
cargo clippy+test (gtk)	4m 21s
cargo test (wasm32)	5m 42s
cargo clippy+test (ubuntu-latest)	1m 21s
cargo clippy+test (wayland)	1m 11s
	40m 8s

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