notification: Add supported-options property

Let applications query supported options for category and button purpose.

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

@@ -63,6 +63,17 @@
       <arg type="s" name="id" direction="in"/>
     </method>
 
+    <!--
+        SupportedOptions:
+
+        Some properties in :ref:`org.freedesktop.impl.portal.Notification.AddNotification`
+        may have options advertised by the notification server.
+
+        The format is the same as for ``SupportedOptions`` property of
+        :ref:`org.freedesktop.portal.Notification`.
+    -->
+    <property name="SupportedOptions" type="a{sv}" access="read"/>
+
     <!--
         ActionInvoked:
         @app_id: App id of the application

data/org.freedesktop.portal.Notification.xml

@@ -393,6 +393,25 @@
       <arg type="s" name="id" direction="in"/>
     </method>
 
+    <!--
+        SupportedOptions:
+
+        Some properties in :ref:`org.freedesktop.portal.Notification.AddNotification`
+        may have options advertised by the server.
+
+        Currently there are the following options the notification server can advertise:
+
+        * ``category`` (``as``)
+
+          Categories that the notification server understands and supports.
+
+        * ``button-purpose`` (``as``)
+
+          Button purposes that the notification server understands and supports.
+          This is the list of options that can be used as `purpose` for `buttons`.
+    -->
+    <property name="SupportedOptions" type="a{sv}" access="read"/>
+
     <!--
         ActionInvoked:
         @id: the application-provided ID for the notification

src/notification.c

@@ -1216,6 +1216,9 @@ static void
 notification_init (Notification *notification)
 {
   xdp_dbus_notification_set_version (XDP_DBUS_NOTIFICATION (notification), 2);
+  g_object_bind_property (G_OBJECT (impl), "supported-options",
+                          G_OBJECT (notification), "supported-options",
+                          G_BINDING_SYNC_CREATE);
 }
 
 static void