Start porting code to python3.

In order to do the port, I have to use gi.repository instead of pygtk. So, the
application is using Gtk3. The application runs with python3 with errors:
- AttributeError: 'gtk.gdk.X11Window' object has no attribute 'new_gc'
- Unknown event <enum GDK_2BUTTON_PRESS of type GdkEventType>

Moreover, it doesn't run as intended. The pdf is not displayed until you press a key.
In addition, you must press a key many times in order for a slide to show up or to change,
it seems that the number of the current page is not working properly. I guess it is linked
with the transissions.

The timer looks OK.
Remove last error

Still no icons
Cleaning import

- Removed commented lines of pygtk
- Imports from gi.repository are cleaner
Fix shebang: python -> python3
Fix syntax

The code is more pythonic
Attempt to load program icon

Fix icon import in util.py but set_icon_list make the program terminate with a
core dumped.
Fix TypeError in pixbufcache
Update FIXME
Correct ShadowType
Cache is not functionnal

Disable the take of pixbuf from cache since it does not work properly.
Fix method and property

Some property were still from GTK2. We now use the ones from GTK3.
Disable swich screensaver status
Slide preview is working again.

The problem is the size of these drawing area which is 1 by default. I set them
manualy to a value so they are visible, but if you resize the window, they don't
resize. It needs fixing.
Fix "goto with label" functionnality.

We can again use the current page label to navigate in the pdf.
'with Gdk.lock()' seems deprecated and unneeded.
Error on my part. Gdk is not deprecated, only some of its functions are.
Fix README
Rename main.py to bin/pympress.
Remove unuseful import.
Modify setup entry point.

We renamme bin/pympress to main.py in order to easily test the programm.
We change the entry points in setup.py so that main.py is installed as
bin/pympress.