Update all gtk crates to 18.x #5764

Job	Run time
cargo clippy+test (windows-latest)	2m 20s
cargo clippy+test (macos-latest)	2m 59s
cargo clippy+test (ubuntu-latest)	1m 20s
mdbook build	14s
druid doctests	1m 26s
Docs (windows-latest)	1m 22s
Docs (macos-latest)	1m 2s
cargo fmt	14s
cargo clippy+test (gtk)	1m 5s
Docs (ubuntu-latest)	1m 27s
cargo test (wasm32)	2m 20s
cargo clippy+test (wayland)	1m 11s
	17m 0s

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