Add ability to set the default orderBy in nested element indexes

[ryansupercool]

We may be missing something but there doesn't appear to be a way to set the default orderBy within nested element? (We're using the ElementIndexViewMode::Table view.

We've defined defineSortOptions which has one attribute but Craft looks to be using the ID instead. Defining defineSources with a "*" source and defaultSort also does not appear to work.

The only way that I can see of setting a default sort order is via the sortOrder column in elements_owners. For nested elements that aren't manually "sortable" is would be really helpful to be able to define the default sort via other means than having to maintain sortOrder for date attributes - this itself may not be the most efficient as from what I can tell you'd have to do this via NestedElementManager::EVENT_AFTER_SAVE_ELEMENTS which will have happened after Craft has already resorted them.