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/sessionMode.js

@@ -0,0 +1,25 @@
+// -*- mode: js; js-indent-level: 4; indent-tabs-mode: nil -*-
+
+const Lang = imports.lang;
+const Shell = imports.gi.Shell;
+
+const Params = imports.misc.params;
+
+const DEFAULT_MODE = 'user';
+
+const _modes = {
+    'gdm': { sessionType: Shell.SessionType.GDM },
+
+    'user': { sessionType: Shell.SessionType.USER }
+};
+
+const SessionMode = new Lang.Class({
+    Name: 'SessionMode',
+
+    _init: function() {
+        let params = _modes[global.session_mode];
+
+        params = Params.parse(params, _modes[DEFAULT_MODE]);
+        Lang.copyProperties(params, this);
+    }
+});