UserSettings Default Sort Column - cascade of default sort, can only have 1

Problem 1: At the moment you can only sort by 1 column, would be useful to be able to sort by 2 (eg chrom/pos) - maybe this would make a good default to VariantGrid?

Problem 2: User settings now cascade, this means there are a few issues with default sort column being a CustomColumn

* Autocomplete forward of column (to restrict to that) doesn't work with cascading
* You could set a sort by column to something that doesn't exist in a later overridden columns collection