Port to QT

[fsimonis]

I am considering to port the application to QT as using GTK has some severe downsides:

• Barely existent documentation for python bindings
• Inconsistently packaged pygobject among distros making it tricky to install
• xdot (which I use for the dot preview) not being maintained any more and locking us into gtk3

Upsides of QT:

• Very good documentation for the python bindings
• Consistently packaged single dependency

Loosing xdot:

• Render to svg/pdf and display it in a Qt window.
• No node highlighting
• Not sure about mouse-over hints
• More functionality possible via xdot conversion, but this requires implementing many things from scratch. Not worth it for the extra functionality.