Fix window list in custom-extension example

The window list in the compositor wasn't updated when a client window
appeared, because itemList had type var and assignment (w/o prior
.slice() call) wouldn't update it.

While at it, modernized QML and resolved issues (unqualified access,
structure, unused code, etc.).

Pick-to: 6.12 6.11
Change-Id: I8712f5c42ae4ec91b70f1c149134211463928681
Reviewed-by: Robert Griebl <robert.griebl@qt.io>