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/qt/listctrl.h

@@ -279,7 +279,7 @@ public:
     virtual int OnGetItemColumnImage(long item, long column) const;
 
     // return the attribute for the given item and column (may return NULL if none)
-    virtual wxListItemAttr *OnGetItemColumnAttr(long item, long WXUNUSED(column)) const
+    virtual wxItemAttr *OnGetItemColumnAttr(long item, long WXUNUSED(column)) const
     {
         return OnGetItemAttr(item);
     }