Turn off FocusOnNavigationEnabled on Qt 5.9

This way we get the same behavior with Qt 5.9 and 5.10 at least, leaving only
5.7 if we pretend that 5.8 never existed.

qutebrowser/browser/webengine/webenginesettings.py

@@ -187,8 +187,17 @@ class ProfileSetter:
         """Initialize settings on the given profile."""
         self.set_http_headers()
         self.set_http_cache_size()
-        self._profile.settings().setAttribute(
+
+        settings = self._profile.settings()
+        settings.setAttribute(
             QWebEngineSettings.FullScreenSupportEnabled, True)
+        try:
+            settings.setAttribute(
+                QWebEngineSettings.FocusOnNavigationEnabled, False)
+        except AttributeError:
+            # Added in Qt 5.8
+            pass
+
         if qtutils.version_check('5.8'):
             self.set_dictionary_language()