Update all gtk crates to 18.x #5753

Job	Run time
cargo clippy+test (wayland)	2m 20s
cargo clippy+test (macOS-latest)	1m 1s
cargo clippy+test (gtk)	2m 21s
cargo clippy+test (windows-2019)	1m 1s
Docs (macOS-latest)	2m 28s
cargo clippy+test (ubuntu-latest)	57s
druid doctests beta	1m 28s
druid doctests	2m 8s
Docs (windows-2019)	2m 57s
Docs (ubuntu-latest)	4m 20s
cargo fmt	11s
mdbook build	30s
cargo clippy+test beta	4m 39s
cargo test (wasm32)	2m 3s
	28m 24s

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