Convert to Python3 and GTK3

I was able to get a working Python3 and GTK3 version of pympress. All the main functionnalities are working according to my tests. However, there is still work to do.

What does not work yet:
* the cache: I was compelled to disable it
* if you resize the preview window, the preview slides are not resized correctly
* the icons cannot be added. If I try to, the program segfault for a reason I cannot figure.