Update all gtk crates to 18.x #5754

Job	Run time
cargo clippy+test (gtk)	1m 16s
cargo fmt	12s
druid doctests beta	1m 5s
Docs (macOS-latest)	1m 19s
druid doctests	1m 23s
Docs (windows-2019)	1m 39s
mdbook build	14s
Docs (ubuntu-latest)	1m 43s
cargo clippy+test beta	3m 2s
cargo clippy+test (macOS-latest)	5m 13s
cargo test (wasm32)	2m 7s
cargo clippy+test (windows-2019)	4m 18s
cargo clippy+test (wayland)	1m 23s
cargo clippy+test (ubuntu-latest)	2m 58s
	27m 52s

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