Don't give keyboard focus to tab bar

This partially solves #7820. The web view still loses focus for an unknown
reason (apparently when swtiching out the rendering process?), but at least it
regains focus now when the window is unfocused and then focused again.

qutebrowser/mainwindow/tabwidget.py

@@ -396,6 +396,7 @@ class TabBar(QTabBar):
         self._win_id = win_id
         self._our_style = TabBarStyle()
         self.setStyle(self._our_style)
+        self.setFocusPolicy(Qt.FocusPolicy.NoFocus)
         self.vertical = False
         self._auto_hide_timer = QTimer()
         self._auto_hide_timer.setSingleShot(True)