Bump thirdparty/SingleApplication from `494772e` to `a8da87d` #95

Job	Run time
build (macos-latest, mac, 5.15.2)	3m 23s
build (windows-latest, windows, 5.12.12, 73)	2m 25s
build (ubuntu-latest, linux, 5.12.12)	1m 15s
build (ubuntu-latest, linux, 6.2.*, qtmultimedia)	1m 15s
	8m 18s

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