Port to GTK3

[Thor77]

[jnumm]

FYI, Debian is in the process of removing pygtk: https://bugs.debian.org/885139

[Thor77]

@jnumm Thank's for the information. Until now I didn't really touch the GUI code in any way and it's probably not working properly with my recent changes.
I'll take a look at PyGObject, which seems to be the way to go for GObject with Python.
I'm not sure if the current Debian Maintainer is active in development anymore in any way, as well, as the Sourceforge-Repo didn't see any change since 2013.