Fix system tray, clean it up

At first I noticed it wasn't working like it should, then realised it
was using APIs that are largely unadvised. Also I saw there's some kind
of system to render numbers in there but this is also something we
generally do not do. There's better ways to display such information
(e.g. using the com.canonical.Unity interface), so I wouldn't bother
having ad-hoc code in there. And if we need to have it, we better have
it in KNotifications.
It now will also use the icon from the theme.

src/controller.cpp

@@ -61,8 +61,7 @@ Controller::Controller(QObject *parent)
 #ifndef Q_OS_ANDROID
     TrayIcon *trayIcon = new TrayIcon(this);
     connect(trayIcon, &TrayIcon::showWindow, this, &Controller::showWindow);
-    trayIcon->setVisible(true);
-    trayIcon->setIconSource(":/assets/img/icon.png");
+    trayIcon->setIconSource("neochat");
     trayIcon->setIsOnline(true);
 #endif