owhierarchicalclustering: Fix performance on deselection

The code had quadratic time complexity in the number of deselected items.
Fixed by a more judicious placement of calls to `_re_enumerate_selections`

Orange/widgets/unsupervised/owhierarchicalclustering.py

@@ -439,6 +439,7 @@ def select_item(self, item, state):
         if item in self._selection:
             if state == False:
                 self._remove_selection(item)
+                self._re_enumerate_selections()
                 self.selectionChanged.emit()
         else:
             # If item is already inside another selected item,
@@ -455,11 +456,11 @@ def select_item(self, item, state):
 
             if state:
                 self._add_selection(item)
-                self._re_enumerate_selections()
 
             elif item in self._selection:
                 self._remove_selection(item)
 
+            self._re_enumerate_selections()
             self.selectionChanged.emit()
 
     def _add_selection(self, item):
@@ -499,8 +500,6 @@ def _remove_selection(self, item):
 
         del self._selection[item]
 
-        self._re_enumerate_selections()
-
     def _selected_sub_items(self, item):
         """Return all selected subclusters under item."""
         def branches(item):