Update all gtk crates to 18.x #5756

Job	Run time
mdbook build	22s
druid doctests	1m 55s
druid doctests beta	1m 53s
Docs (macOS-latest)	1m 40s
cargo fmt	17s
Docs (windows-2019)	1m 45s
cargo clippy+test beta	3m 9s
cargo clippy+test (macOS-latest)	4m 5s
Docs (ubuntu-latest)	2m 33s
cargo clippy+test (gtk)	2m 24s
cargo clippy+test (windows-2019)	2m 56s
cargo test (wasm32)	5m 31s
cargo clippy+test (ubuntu-latest)	2m 59s
cargo clippy+test (wayland)	3m 58s
	35m 27s

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