Delete QTextDocument properly in completion.

The CompletionItemDelegate gets reused by Qt for various items in the
completion. Every time _get_textdoc() was called we created a new
QTextDocument, but since it has a long-living parent set (the delegate)
the old one was never actually garbage collected.

We now explicitly delete the old QTextDocument as it's not needed
anymore by either Qt or Python.

See #1476.

qutebrowser/completion/completiondelegate.py

@@ -183,6 +183,8 @@ class CompletionItemDelegate(QStyledItemDelegate):
         text_option.setAlignment(QStyle.visualAlignment(
             self._opt.direction, self._opt.displayAlignment))
 
+        if self._doc is not None:
+            self._doc.deleteLater()
         self._doc = QTextDocument(self)
         self._doc.setDefaultFont(self._opt.font)
         self._doc.setDefaultTextOption(text_option)