docs/notification: Write XML interface definition for version 2

This takes the discussion in
flatpak#983 and turns it
into an actual possible v2 of the notification interface.

The permission part was intentionally left out since it's tangential to
the new properties and features proposed here.

data/org.freedesktop.impl.portal.Notification.xml

@@ -48,6 +48,20 @@
       <annotation name="org.qtproject.QtDBus.QtTypeName.In2" value="QVariantMap"/>
       <arg type="a{sv}" name="notification" direction="in"/>
     </method>
+
+    <!--
+        ListSupportedProperties:
+        @supported_properties: A list of supported properties and their supported values, if applicable
+
+        Returns a list of supported properties with
+        specific values if applicable for the property.
+
+        The format of the @supported_properties is the same as for
+        :ref:`org.freedesktop.portal.Notification.ListSupportedProperties`.
+    -->
+    <method name="ListSupportedProperties">
+      <arg type="a{sv}" name="supported_properties" direction="out"/>
+    </method>
     <!--
         RemoveNotification:
         @app_id: App id of the application