Always set FocusOnNavigationEnabled

This fixes some focus issues after Qt 5.11 changes. There might be better ways
to solve them, but for now, this will work.

See https://codereview.qt-project.org/#/c/221408/10 and #3661:
https://github.com/qutebrowser/qutebrowser/issues/3661#issuecomment-375969315

Might also negatively affect #3834 as it essentially reintroduces QTBUG-52999 on
any Qt version: https://bugreports.qt.io/browse/QTBUG-52999

Might be reverted at a later date, but for now, I want an easy way to make tests
work on Qt 5.11 to spot further issues.

qutebrowser/browser/webengine/webenginesettings.py

@@ -176,12 +176,24 @@ class ProfileSetter:
         """Initialize settings on the given profile."""
         self.set_http_headers()
         self.set_http_cache_size()
-        self._profile.settings().setAttribute(
-            QWebEngineSettings.FullScreenSupportEnabled, True)
+        self._init_attributes()
+
         if qtutils.version_check('5.8'):
             self._profile.setSpellCheckEnabled(True)
             self.set_dictionary_language()
 
+    def _init_attributes(self):
+        """Initialize hard-coded attributes."""
+        values = {
+            'FullScreenSupportEnabled': True,
+            'FocusOnNavigationEnabled': True,
+        }
+        settings = self._profile.settings()
+        for name, value in values.items():
+            attr = getattr(QWebEngineSettings, name, None)
+            if attr is not None:
+                settings.setAttribute(attr, value)
+
     def set_http_headers(self):
         """Set the user agent and accept-language for the given profile.