Replace wxTreeItemAttr and wxListItemAttr with wxItemAttr

The two existing structs were completely identical, just replace them with a
single wxItemAttr.

Notice that wxDataViewItemAttr is not quite the same, although pretty similar,
so it remains separate for now. It would be nice to combine it with this one
too in the future, e.g. to make it simpler to make items bold in a wxListCtrl.

include/wx/msw/treectrl.h

@@ -34,7 +34,8 @@ class  WXDLLIMPEXP_FWD_CORE wxDragImage;
 struct WXDLLIMPEXP_FWD_CORE wxTreeViewItem;
 
 // hash storing attributes for our items
-WX_DECLARE_EXPORTED_VOIDPTR_HASH_MAP(wxTreeItemAttr *, wxMapTreeAttr);
+class wxItemAttr;
+WX_DECLARE_EXPORTED_VOIDPTR_HASH_MAP(wxItemAttr *, wxMapTreeAttr);
 
 // ----------------------------------------------------------------------------
 // wxTreeCtrl