Allow rows to be added and removed from ranked variants

[AlistairNWard]

For example, someone may want to have the coverage as a row in the table for the purposes of prioritizing the variants. We should probably have a generic glyph for non-default rows.

Also, as we include custom annotations, people will want to include these in the ranked variants, e.g. conservation for Tomasz. Can we sort genes based on these updated ranked variants prioritizations?

[AlistairNWard]

Also drag and drop reordering, rather than many clicks to move a property up the table.