Set focus proxy for AbstractTab

By default, the AbstractTab object got the focus, which means things
like key events passed to it didn't actually get passed through to the
web view, causing these tests to fail:

    tests/end2end/features/test_keyinput_bdd.py::test_forwarding_all_keys
    tests/end2end/features/test_keyinput_bdd.py::test_forwarding_special_keys

Now we make sure the real underlying WebView always gets the keyboard
focus.

qutebrowser/browser/tab.py

@@ -427,6 +427,7 @@ class AbstractTab(QWidget):
         self.search._widget = widget
         widget.mouse_wheel_zoom.connect(self.zoom.on_mouse_wheel_zoom)
         widget.setParent(self)
+        self.setFocusProxy(widget)
 
     @property
     def cur_url(self):