Draft: lowercase use_link option for arrays

Inside the class `_DraftLink` the `onDocumentRestored` function
tests for the existence of the old attribute `useLink`. If it is
present, it uses it to define the new variable name `use_link`.
This is done use Python naming conventions.
The old `useLink` is deleted so it is not longer saved
together with the object when the document is saved.

src/Mod/Draft/draftobjects/polararray.py

@@ -38,5 +38,5 @@ def make_polar_array(obj,
     """
     obj = Draft.makeArray(obj,
                           arg1=center, arg2=angle, arg3=number,
-                          useLink=use_link)
+                          use_link=use_link)
     return obj