Integrate DesktopNotification into system tray/status notifier

Since the DBus notification spec is quite widespread by now, and in fact KDE's
StatusNotifierItem uses that same spec for displaying popup messages, we now
integrate the former DesktopNotification into StatusNotifierItem.

If DBus or a NotificationsClient are not present, we fall back to traditional systray
popups. This could be fugly if we have StatusNotifier, but not NotificationsClient, but
all platforms supporting the former should also support the latter by now.

Having this in SystemTray also makes our notification handling code much nicer, and we have
one less backend to care about. Additionally, users using current versions of KDE or GNOME
will get pretty popups automatically.