Port to GTK+ 3

[quozl]

GTK+ 2 is becoming old, though it is still available in most distributions.

A port to GTK+ 3 may help in future.

https://github.com/sugarlabs/irc-activity contains a port made in 2014 from older and modified urk sources.

[madewokherd]

More importantly this is a prerequisite to a Python 3 port, and Python 2 is no longer broadly available.

Unfortunately, it complicates setup on Windows, since gtk3 doesn't have a convenient installer like pygtk. Hopefully something can be put together with fedora's mingw packages or failing that msys2.

[madewokherd]

WIP on this branch: https://github.com/madewokherd/urk/tree/gtk3