Feature: Add sorting by _display_value

[robxbob]

Currently sorting only works on existing columns, it would be nice if we could sort by _display_value or better yet default the sort to _display_value on relationship types.

For example if we had a Book-Author relationship, when sorting on sort=[author], it would sort by _display_value by default. Currently, an uncaught error occurs when attempting to sort directly.

[robxbob]

Also for filters as it would allow generalized functions that need the "primary field" to filter by. The generalized function could default to _displayValue so that a filter can look like: filter=[[ { path: "_display_value", op: "like", value: inputValue } ]]