Cleanly separate rendering and event handling

Cleanly separate rendering and event handling

Moving to glium 0.26 / glutin 0.22 required adopting a new event loop
stlye, which in turn was the right excuse for a long overdue code
rework. Some things still don't work quite right at this point,
FPS computation is incorrect and some keyboard commands are still
commented out... But hey, the code looks much nicer now!