Add line color / grade value mapping

[dorthrithil]

Add a mapping from a specific grad value to a default line color. This could replace the current system wide default. It may be tricky to combine it with the color attribute of the line that @BlobbyBob is currently implementing, as it will get confusing when the two options are used together (e.g. is the line yellow because the color was chosen to match the hold color or is it yellow to match the grade?). So maybe an additional toggle to enable this and disable the line color attribute is needed.