WIP: convert manual-example.ui to Gtk3 #744

Job	Run time
rip-and-test-clang	18m 9s
rip-and-test	18m 14s
htmldocs	26m 52s
package-arch (debian:buster)	21m 31s
package-indep (debian:buster)	49m 8s
package-arch (debian:bullseye)	22m 39s
package-indep (debian:bullseye)	47m 13s
package-arch (debian:bookworm)	25m 35s
package-indep (debian:bookworm)	42m 6s
package-arch (debian:sid)	23m 47s
package-indep (debian:sid)	42m 50s
	5h 38m 4s

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