GtkDialog is too flexible in terms of UI (headerbars vs action bar,
etc), and has archaic APIs. It is time to retire it.

Replace GtkComboBoxText with GtkDropDown+GtkStringList.

For now, just ignore deprecations. Eventually,
we will have to go through, drop demos that are
for wholly deprecated widgets, and update others.

This shows how to use a layout manager in a widget,
implemented in javascript. The example sets up the
environment for running from the toplevel dir, assuming
that the build dir is called 'build'.

Amend LD_PRELOAD instead of replacing it, so you can just
point it at the right libgtk-4.so while running squares.py
from elsewhere.

This shows how to do custom drawing in a widget,
implemented in python. The example sets up the
environment for running from the toplevel dir,
assuming that the build dir is called 'build'.

Simplify the code a bit and make the window
match the existing screenshot.