Update all gtk crates to 18.x #5749

Job	Run time
mdbook build	1m 52s
Docs (macOS-latest)	1m 19s
druid doctests beta	2m 57s
druid doctests	2m 23s
Docs (windows-2019)	1m 26s
cargo fmt	11s
Docs (ubuntu-latest)	1m 20s
cargo clippy+test beta	4m 40s
cargo clippy+test (macOS-latest)	1m 16s
cargo clippy+test (gtk)	1m 15s
cargo clippy+test (windows-2019)	1m 13s
cargo test (wasm32)	8m 14s
cargo clippy+test (wayland)	1m 22s
cargo clippy+test (ubuntu-latest)	1m 9s
	30m 37s

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