Make it possible to disable/enable the notification area icon

The GNOME HIG states that:

    * Only core GNOME programs may perpetually display an icon in the status area.
    * Non-core programs for which a perpetual icon may be useful must default to not perpetually showing the icon. Users may select to enable a perpetual icon for the application as a preference.

I hereby suggest to move the hard-coded status icon code from gui.py to a plugin. Enabling it would instantly add the LottaNZB icon to the notification area. Additionally, the "Close" menu entry would be added to the file menu. If the plugin is not enabled, closing the LottaNZB main window would entirely shut down LottaNZB. I guess the plugin should be enabled by default though, ignoring the fact that LottaNZB is not a core GNOME application. ;-)

Another thing to think about: What happens to the "Start minimized" option? Since it makes only sense if this plugin is enabled, it would need to be managed by the plugin, but we cannot add a plugin tab containing a single checkbox imho. We could either try to hook into the "General" tab somehow and add it there if the plugin is enabled (dirty hacks unavoidable ;-) or we simply get rid of it.

Basically, I'd just like to do what the Banshee developers did.