Draft: Modernize and Port demos/examples to GTK4

This MR tries to port all the files in examples folder to GTK4. It also tries to replace the use of deprecated stuff with non-deprecated stuff.

All GTK3 stuff has been renamed appropriately.

Ported Files:

• cairo-demo.py (as demos/cairo-gtk4.py)

Modernized Files:

• cairo-demo.py (renamed to demos/cairo-gtk3.py now)
• demo/demo.py (renamed to demos/gtk3.py now)
• options.py
• properties.py
• signals.py

Files that still need a port:

• demo/demo.py (renamed to demos/gtk3.py now)
• demo/demos (renamed to demos/gtk3_demos now)

See commit messages for more details.