Test for #3970.

* This shows the problem in Haiku, it is not easily possible to change the
color of BStringItem.
* It also shows that the BeOS implementation doesn't restore the view
state after drawing items, so the last item in the list also draws
green, and selecting the first also makes it green.
Set item colors in BListView instead of BStringItem

* Fixes #3970 without introducing the bugs from BeOS
* Makes it easy to override BStringItem just to change the text color.
* Makes it easy to implement custom list items using the correct colors.