Gtk 3

This pull request is intended to get rid of GTK depricates and make the GTK code compatible to GTK3.

I can successfully compile and run this code with GTK2.
But with GTK3 I can just successfully compile the code but if I try to run it I get the error:
```
# ettercap -G 

ettercap 0.8.0 copyright 2001-2014 Ettercap Development Team


GTK support is not compiled in ettercap

```
I think now @LocutusOfBorg need to get it linked properly.
After that we make the compilation and linking against libgtk3 choosable by the user.