Detect session lock on Xfce/XUbuntu 16.04

The current code does not work with XUbuntu 16.04, which uses
light-locker to lock the screen.

I am not using a screensaver, but I suspect the screensaver activation
may have been detected by the existing events, and using it could thus
mask the issue.

src/core/ScreenLockListenerDBus.cpp

@@ -20,6 +20,7 @@
 #include <QDBusConnection>
 #include <QDBusInterface>
 #include <QDBusReply>
+#include <QProcessEnvironment>
 
 ScreenLockListenerDBus::ScreenLockListenerDBus(QWidget *parent):
     ScreenLockListenerPrivate(parent)
@@ -51,6 +52,15 @@ ScreenLockListenerDBus::ScreenLockListenerDBus(QWidget *parent):
                 this, //receiver
                 SLOT(logindPrepareForSleep(bool)));
 
+    QString sessionId = QProcessEnvironment::systemEnvironment().value("XDG_SESSION_ID");
+    systemBus.connect(
+                "", // service
+                QString("/org/freedesktop/login1/session/") + sessionId, // path
+                "org.freedesktop.login1.Session", // interface
+                "Lock", // signal name
+                this, //receiver
+                SLOT(unityLocked()));
+
     sessionBus.connect(
                 "com.canonical.Unity", // service
                 "/com/canonical/Unity/Session", // path