Add SingleMainWindow key to desktop entry #1009

Job	Run time
build-flatpak	4m 10s
build-rpm (fedora:38, gcc, 5)	3m 43s
build-macos (arm64)	5m 53s
build-rpm (fedora:39, gcc, 5)	3m 36s
build-deb (debian:11)	2m 58s
build-macos (x64)	6m 38s
build-rpm (opensuse/tumbleweed:latest, gcc, 5)	3m 0s
build-windows-mingw	11m 38s
build-deb (debian:12)	3m 8s
build-rpm (fedora:38, clang)	3m 40s
build-deb (ubuntu:22.04)	2m 56s
build-rpm (fedora:39, clang)	3m 39s
build-deb (ubuntu:23.10)	3m 48s
reuse-lint	14s
build-rpm (opensuse/tumbleweed:latest, 6)	3m 41s
build-windows-msvc (msvc) / build-windows-msvc	14m 50s
	1h 17m 32s

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