Support itemMapper instead of textMapper

[giorgenes]

The textMapper property only maps text -> text, it would be great if instead it could map value -> Widget, this way we could display arbitrary widgets instead of just text.

I have a use case for this witch is to annotate the value. I checked the code it seems straightforward to support this.

I could give it a go if you're open to a merge request.

[thecodepapaya]

I have a similar use case where I need to give custom decoration to unselected text. Having an itemMapper instead of textMapper would really help.