gtk overrides: Make GTK4 widgets iterable

GTK4 removed the Gtk.Container interface, and added API on Gtk.Widget
to iterate over children instead.

Making Widgets iterable allows to easily loop over the children and
check is a widget is a child of an other one.

A new test is also added.
11 jobs for wip/jfelder/gtk4-widget-iterable in 15 minutes and 9 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build And Test
passed #999814
flatpak
gnome-master

00:01:34

passed #999815
flatpak
gnome-master-gtk4

00:01:51

passed #999806
win32-ps
mingw32

00:02:44

passed #999807
win32-ps
mingw64

00:02:46

passed #999813
old-i386-py3

00:02:23

failed #999812
allowed to fail
pypy3.6

00:02:02

passed #999808
python3.6

00:03:30

passed #999809
python3.7

00:06:11

passed #999810
python3.8

00:04:41

passed #999811
python3.9

00:07:30

 
  Coverage
passed #999816
coverage

00:02:08

 
Name Stage Failure
failed
pypy3.6 Build And Test
Ok:                 0   
Expected Fail: 0
Fail: 1
Unexpected Pass: 0
Skipped: 0
Timeout: 0

Cleaning up file based variables
ERROR: Job failed: exit code 1