Update all gtk crates to 18.x #5751

Job	Run time
druid doctests	1m 21s
mdbook build	14s
Docs (macOS-latest)	2m 2s
druid doctests beta	51s
Docs (windows-2019)	3m 2s
Docs (ubuntu-latest)	5m 20s
cargo fmt	15s
cargo clippy+test beta	2m 57s
cargo clippy+test (macOS-latest)	1m 29s
cargo clippy+test (windows-2019)	1m 31s
cargo clippy+test (gtk)	1m 13s
cargo clippy+test (wayland)	1m 11s
cargo clippy+test (ubuntu-latest)	1m 37s
cargo test (wasm32)	3m 16s
	26m 19s

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