Beautify MacVim's dmg installer background and volume icon #856

Job	Run time
build-and-test (macos-13, 15.2, true, vimtags, check-xcodeproj-compat)	20m 42s
build-and-test (macos-14, 15.4, false, true)	23m 13s
build-and-test (macos-15, 16.2, true, true, true)	27m 18s
	1h 11m 13s

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