Delegate mode information to a dedicated object

Rather than accessing global.session_type / global.session_mode
all over the place, delegate mode information to a dedicated
sessionMode object. While not very useful for now, we will replace
checks for a particular mode with checks for particular properties
that sessionMode defines based on global.session_mode.

https://bugzilla.gnome.org/show_bug.cgi?id=676156

js/ui/popupMenu.js

@@ -889,7 +889,7 @@ const PopupMenuBase = new Lang.Class({
 
     addSettingsAction: function(title, desktopFile) {
         // Don't allow user settings to get edited unless we're in a user session
-        if (global.session_type != Shell.SessionType.USER)
+        if (Main.sessionMode.sessionType != Shell.SessionType.USER)
             return null;
 
         let menuItem = this.addAction(title, function() {