Basic StatusNotifierItem support

This implements the new dbus spec that replaces the old systray:
<http://www.notmart.org/misc/statusnotifieritem/>

Until the spec has been ratified by freedesktop.org (which should happen soon), we're
using the org.kde namespace to get it working in newish (>= 4.4) KDE versions at least.
In order to support this even without KDE integration, we've taken a bunch of code
from KDE's implementation (written by Marco Martin), only using the parts that are
relevant for Quassel, and adapting them to our needs.

In case no DBus support is present, or noone listens for the service, we fall back
to the classical tray icon.

Various bits and pieces are still missing, e.g. tooltips, and the context menu
isn't working yet if Qt doesn't support QSystemTrayIcon (but you'll get the dbus
item if there's a service listening even without tray icon support in Qt).