reduce spacing between legend items #459

Job	Run time
build-macos-brew (clang-9)	2m 53s
build-macos-no-brew-Qt6 (clang-9)	5m 10s
build-linux (gcc-9)	6m 25s
build-linux-Qt6 (gcc-9)	8m 8s
build-macos-brew-Qt6 (clang-9)	3m 12s
build-macos-no-brew (clang-9)	4m 52s
build-windows-mingw (5.15.*)	6m 51s
build-windows-mingw-Qt6 (6.3.0, qtmultimedia qt5compat)	10m 43s
build-windows-vs (5.15.*)	6m 49s
build-windows-vs-Qt6 (6.3.0, qtmultimedia qt5compat)	12m 21s
	1h 7m 24s

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