Revert "xapp-sn-watcher: Don't enable in MATE"

This reverts commit b6b8564.

Tested in 24.04.

The following applets do not support SNI:

- mate-indicator-applet
- mate-indicator-applet-complete
- mate-panel/notification-area-applet

mate-xapp-status-applet only supports it if xapp-sn-watcher is running,
and it won't if MATE isn't in this list.

Note: Ayatana indicators do support SNI but they're not used in MATE
by default. mate-xapp-status-applet is recommended instead. If people
want to use Ayatana indicators, they need to remove MATE from this list
so the two do not conflict.

Author: clefebvre

schemas/org.x.apps.gschema.xml

@@ -25,7 +25,7 @@
        </description>
      </key>
      <key name="status-notifier-enabled-desktops" type="as">
-       <default>["Cinnamon", "X-Cinnamon"]</default>
+       <default>["MATE", "Cinnamon", "X-Cinnamon"]</default>
        <summary>List of desktops to run the XApp StatusNotifierWatcher service on..</summary>
        <description>The name in the session's XDG_CURRENT_DESKTOP environment variable is checked against this list.  If the name is here, the service is started to allow XApp status applets to support the StatusNotifer spec (this includes appindicator).
        </description>