Improve performance when adding new history item.

Fixes #919.

There were two issues here:

- CompletionWidget didn't delete the old model when setting a new one. This
  means filterAcceptsRow was called for models which aren't even used anymore.

- setChild was used instead of appendRow for the BaseCompletionModel, which
  caused Qt to call filterAcceptsRow once for every item of the completion
  model instead of only once.

qutebrowser/completion/completionwidget.py

@@ -198,10 +198,16 @@ class CompletionView(QTreeView):
         Args:
             model: The model to use.
         """
+        old_model = self.model()
         sel_model = self.selectionModel()
+
         self.setModel(model)
+
         if sel_model is not None:
             sel_model.deleteLater()
+        if old_model is not None:
+            old_model.deleteLater()
+
         for i in range(model.rowCount()):
             self.expand(model.index(i, 0))
 

qutebrowser/completion/models/base.py

@@ -88,16 +88,15 @@ class BaseCompletionModel(QStandardItemModel):
         assert not isinstance(name, int)
         assert not isinstance(desc, int)
         assert not isinstance(misc, int)
+
         nameitem = QStandardItem(name)
         descitem = QStandardItem(desc)
         if misc is None:
             miscitem = QStandardItem()
         else:
             miscitem = QStandardItem(misc)
-        idx = cat.rowCount()
-        cat.setChild(idx, 0, nameitem)
-        cat.setChild(idx, 1, descitem)
-        cat.setChild(idx, 2, miscitem)
+
+        cat.appendRow([nameitem, descitem, miscitem])
         if sort is not None:
             nameitem.setData(sort, Role.sort)
         if userdata is not None: