Update all gtk crates to 18.x #5755

Job	Run time
cargo fmt	16s
cargo clippy+test (gtk)	1m 8s
druid doctests beta	1m 22s
Docs (macOS-latest)	1m 36s
druid doctests	1m 43s
mdbook build	13s
Docs (windows-2019)	1m 34s
cargo clippy+test beta	2m 3s
Docs (ubuntu-latest)	1m 39s
cargo clippy+test (macOS-latest)	6m 27s
cargo test (wasm32)	2m 11s
cargo clippy+test (windows-2019)	3m 26s
cargo clippy+test (wayland)	1m 16s
cargo clippy+test (ubuntu-latest)	3m 4s
	27m 58s

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