places-menu: recognize if apps-menu is present, and place to the right of it

If apps-menu is enabled at the same time as places-menu, the right order
is activities, apps-menu, places-menu, app-menu. Previously we used the
extension activation order to obtain this, but it's not reliable enough,
so make it explicit in code.

extensions/places-menu/extension.js

@@ -106,7 +106,11 @@ let _indicator;
 
 function enable() {
     _indicator = new PlacesMenu;
-    Main.panel.addToStatusArea('places-menu', _indicator, 1, 'left');
+
+    let pos = 1;
+    if ('apps-menu' in Main.panel.statusArea)
+	pos = 2;
+    Main.panel.addToStatusArea('places-menu', _indicator, pos, 'left');
 }
 
 function disable() {