Make notifications work

Based on d1eecf8b97.

Relevant parts from that commit message:

    `QVariant.Type` has moved to `QMetaType.Type`[1][] and QMeta.Type
    doesn't work with int().

    `QtWebEngineCore.QWebEnginePage.Feature` doesn't work with int(), so add
    it to the maps twice.

    [1]: https://www.qt.io/blog/whats-new-in-qmetatype-qvariant

Also:

    `QImage(':/icons/qutebrowser-64x64.png')` yields and empty QImage. This
    is not fixed but things don't crash because of it anymore. For instance
    the "Title & Body" test on https://web-push-book.gauntface.com/demos/notification-examples/

but that part seems to work fine now. I don't quite understand why the original
commit added an "or bool(icon.rect())", but I can't find anything breaking
things without.

qutebrowser/browser/webengine/notification.py

@@ -733,7 +733,15 @@ class _ServerCapabilities:
 def _as_uint32(x: int) -> QVariant:
     """Convert the given int to an uint32 for DBus."""
     variant = QVariant(x)
-    successful = variant.convert(QVariant.Type.UInt)
+
+    try:
+        # Qt 5
+        target_type = QVariant.Type.UInt
+    except AttributeError:
+        # Qt 6
+        target_type = QMetaType(QMetaType.Type.UInt.value)
+
+    successful = variant.convert(target_type)
     assert successful
     return variant
 
@@ -993,7 +1001,10 @@ class DBusNotificationAdapter(AbstractNotificationAdapter):
         actions = []
         if self._capabilities.actions:
             actions = ['default', 'Activate']  # key, name
-        return QDBusArgument(actions, QMetaType.Type.QStringList)
+        return QDBusArgument(
+            actions,
+            qtutils.extract_enum_val(QMetaType.Type.QStringList),
+        )
 
     def _get_hints_arg(self, *, origin_url: QUrl, icon: QImage) -> Dict[str, Any]:
         """Get the hints argument for present()."""

qutebrowser/browser/webengine/webenginetab.py

@@ -888,9 +888,11 @@ class _WebEnginePermissions(QObject):
 
     # Using 0 as WORKAROUND for:
     # https://www.riverbankcomputing.com/pipermail/pyqt/2019-July/041903.html
+    # Fixed in PyQt 5.13.1
 
     _options = {
         0: 'content.notifications.enabled',
+        QWebEnginePage.Feature.Notifications: 'content.notifications.enabled',
         QWebEnginePage.Feature.Geolocation: 'content.geolocation',
         QWebEnginePage.Feature.MediaAudioCapture: 'content.media.audio_capture',
         QWebEnginePage.Feature.MediaVideoCapture: 'content.media.video_capture',
@@ -902,6 +904,7 @@ class _WebEnginePermissions(QObject):
 
     _messages = {
         0: 'show notifications',
+        QWebEnginePage.Feature.Notifications: 'show notifications',
         QWebEnginePage.Feature.Geolocation: 'access your location',
         QWebEnginePage.Feature.MediaAudioCapture: 'record audio',
         QWebEnginePage.Feature.MediaVideoCapture: 'record video',