DataRow lay out children like a horizontal StackPanel if DataTable is not present

[9rnsr]

Separate PR from #781

This added behavior just covers a corner case. If a user fails to place corresponding DataTable at correct position, displaying somethings is better than nothing.

Each rows layout their columns independent, because there's no main controller to remember the column widths, and then it looks like a horizontal StackPanel.

[michael-hawker]

Was thinking about this scenario a bit more over the weekend, and I see a couple of directions:

1. If they just have a DataRow it's better to just have another panel there instead if they don't intend to use it as it's intended. Providing another behavior may be confusing or not lead them to understanding that there's more to be done. Probably something to think about if we can indicate this different state another way.

2. If we did want a default behavior. Probably doing a more EqualPanel type behavior where we space each item out based on the available width / number of items would make more sense, as then as long as the developer has the same number of items in each row (which they would with a data template) then it evenly spaces each item and maintains columns between the rows compared to them being jagged.