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/status/keyboard.js

@@ -63,7 +63,7 @@ const XKBIndicator = new Lang.Class({
 
         this._syncConfig();
 
-        if (global.session_type == Shell.SessionType.USER) {
+        if (Main.sessionMode.sessionType == Shell.SessionType.USER) {
             this.menu.addMenuItem(new PopupMenu.PopupSeparatorMenuItem());
             this.menu.addAction(_("Show Keyboard Layout"), Lang.bind(this, function() {
                 Main.overview.hide();