Update all gtk crates to 18.x #5752

Job	Run time
mdbook build	13s
Docs (macOS-latest)	1m 10s
druid doctests	51s
Docs (windows-2019)	3m 0s
druid doctests beta	1m 18s
Docs (ubuntu-latest)	1m 44s
cargo fmt	14s
cargo clippy+test beta	1m 58s
cargo clippy+test (macOS-latest)	8m 10s
cargo clippy+test (gtk)	1m 11s
cargo clippy+test (windows-2019)	5m 21s
cargo test (wasm32)	3m 30s
cargo clippy+test (wayland)	3m 18s
cargo clippy+test (ubuntu-latest)	4m 30s
	36m 28s

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