Port to python3 and pygobject-3.x

As the todo listed python3 I decided to this as a "training". Although in this case it was mostly search and replace.
With these commits it's more or less a 1:1 port. So the same quirks and problems are there (and maybe 1 or 2 more *cough*) Although I planned to work on those. The "less" involves the change of the default background (transparent pictures with black items on a black bg cause a slight problem) and disabling the position saving.
Now to the new issues.
Pressing an unset key result with the py2 version in
  <gtk.gdk.Event at 0xb56a7728: GDK_KEY_PRESS keyval=n>
Now it provides this:
  <void at 0x8fce718>

The other happens with pictures larger than the screen. Now and then scrolling the picture doesn't work.

Well. Enough with the text.
I decided to create the PR as you might be interested in thiss, although the last change is from 2011.