Fix API deprecation warning.

src/multiplatform_widgets/widgets.py

@@ -37,7 +37,7 @@ class list(object):
   if self.system == "Windows":
    if reversed == False: items = self.list.GetItemCount()
    else: items = 0
-   self.list.InsertStringItem(items, item[0])
+   self.list.InsertItem(items, str(item[0]))
    for i in xrange(1, len(self.columns)):
     self.list.SetStringItem(items, i, item[i])
   else: