In a few places I also simplified a few format strings:
-outln("{} item{}", items, items.size() == 1 ? ' ' : 's');
+outln("{} item(s)", items);
In my opinion this is more readable and in some places it incorrectly
wrote '0 item' which is "fixed" now. In other places the placeholder
space looked weird.
This patch introduces the HeaderView class, which is a widget that
implements the column headers of TableView and TreeView.
This greatly simplifies event management in the view implementations
and also makes it much easier to eventually implement row headers.
