Set MainWindow as parent of TabbedBrowser

If we close the MainWindow (and it gets deleted), we need to make sure to delete
the TabbedBrowser as well.

Fixes #3781

qutebrowser/mainwindow/mainwindow.py

@@ -184,7 +184,8 @@ class MainWindow(QWidget):
             private = bool(private)
         self._private = private
         self.tabbed_browser = tabbedbrowser.TabbedBrowser(win_id=self.win_id,
-                                                          private=private)
+                                                          private=private,
+                                                          parent=self)
         objreg.register('tabbed-browser', self.tabbed_browser, scope='window',
                         window=self.win_id)
         self._init_command_dispatcher()

tests/end2end/features/misc.feature

@@ -568,4 +568,4 @@ Feature: Various utility commands.
     Scenario: Simple adblock update
         When I set up "simple" as block lists
         And I run :adblock-update
-        Then the message "adblock: Read 1 hosts from 1 sources." should be shown
+        Then the message "adblock: Read 1 hosts from 1 sources." should be shown