windowMenu: Only add workspace actions when enabled by session mode

When workspaces are disabled by the session mode, all workspace
related actions don't make sense. Worse, the "Move to Workspace ..."
actions allow sending windows into a nirvana with no means to
get them back.

https://bugzilla.gnome.org/show_bug.cgi?id=730653

js/ui/windowMenu.js

@@ -81,7 +81,9 @@ const WindowMenu = new Lang.Class({
             type == Meta.WindowType.SPLASHSCREEN)
             item.setSensitive(false);
 
-        if (!Meta.prefs_get_workspaces_only_on_primary() || window.is_on_primary_monitor()) {
+        if (Main.sessionMode.hasWorkspaces &&
+            (!Meta.prefs_get_workspaces_only_on_primary() ||
+             window.is_on_primary_monitor())) {
             let isSticky = window.is_on_all_workspaces();
 
             item = this.addAction(_("Always on Visible Workspace"), Lang.bind(this, function() {