Make multi-select background & caret colour customizable #42

Job	Run time
build_windows_cmake (Release, x64, amd64)	4m 13s
build_windows_msys2 (Release, x86_64)	6m 3s
build_windows (Release, x64)	5m 50s
build_windows_msys2 (Release, i686)	5m 39s
build_windows_msys2 (Debug, x86_64)	5m 31s
build_windows_msys2 (Debug, i686)	4m 39s
build_windows (Release, Win32)	3m 19s
build_windows (Release, ARM64)	4m 19s
build_windows (Debug, x64)	6m 25s
build_windows (Debug, Win32)	4m 43s
build_windows (Debug, ARM64)	3m 5s
	53m 46s

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