dbus: Add a read-only org.gnome.Shell.Mode property

This is meant to expose the global.session_mode to applications such
as the gnome-tweak-tool, which would need it to differentiate between
the vanilla GNOME Shell mode and the fallback replacement mode.

https://bugzilla.gnome.org/show_bug.cgi?id=689300

js/ui/shellDBus.js

@@ -46,6 +46,7 @@ const GnomeShellIface = <interface name="org.gnome.Shell">
     <arg type="i" direction="in" name="width"/>
     <arg type="i" direction="in" name="height"/>
 </method>
+<property name="Mode" type="s" access="read" />
 <property name="OverviewActive" type="b" access="readwrite" />
 <property name="ShellVersion" type="s" access="read" />
 </interface>;
@@ -186,6 +187,8 @@ const GnomeShell = new Lang.Class({
         flashspot.fire();
     },
 
+    Mode: global.session_mode,
+
     get OverviewActive() {
         return Main.overview.visible;
     },