Begin porting xfce4-notifyd to Gtk3

[flo.xfce at gmx-topmail.de]

Hey,
I started a port of xfce4-notifyd to Gtk3. It is hosted on Github:
https://github.com/Flow-It/xfce4-notifyd/tree/flo/gtk3
I didn't fix deprecations for now, although I started working on that
locally.
Unfortunately this port broke the notification themes. I'm not really
into GtkSytle/GtkStyleContext and the CSS stuff. Theming is quite low on
my priority list.

Regards
Flo