Display markers for signs #130

Job	Run time
fmt	4s
docs	2m 45s
clippy	2m 46s
viewer	7s
build (macos-11, aarch64-apple-darwin)	4m 54s
test (ubuntu-latest, stable)	2m 57s
build (macos-11, x86_64-apple-darwin)	2m 56s
test (windows-latest, stable)	6m 31s
build (windows-2019, x86_64-pc-windows-msvc, .exe)	5m 50s
test (macOS-latest, stable)	4m 39s
build (windows-2019, i686-pc-windows-msvc, .exe)	4m 21s
build (ubuntu-20.04, x86_64-unknown-linux-gnu)	3m 19s
	41m 9s

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