Allow setting selection for DetailedList

### What is the problem or limitation you are having?

I was working on an app with a DetailedList and needed to rearrange some items. I added a few buttons to move around selected items by removing them from the ListSource and re-inserting them.

The problem arose when I tried to get them re-selected. The DetailedList interface has the "selected" attribute that represents which row is selected, but unlike the Selection widget, it cannot be written to select a row from the list.

### Describe the solution you'd like

Add a setter for DetailedList.selection similar to Selection.value.

### Describe alternatives you've considered

DetailedList could have a method to select a row.

### Additional context

Related:
* https://github.com/beeware/toga/issues/2702

Start at the DetailedList entry point and compare its read-only selection behavior with Selection.value. Implement an assignable DetailedList.selection consistent with that comparison, and consider the related issue #2702; done means callers can re-select rows after removing and reinserting items.

Written by the indexing model from the issue text.