core: make Window.close() return whether the close was accepted

[tilladam]

Previously Window.close() returned true for any top-level window regardless
of the outcome, so callers could not tell whether the close-requested
callback actually let the window close.

WindowItem::close() now returns the result of the close request:

• true when the callback accepted the close (the window is hidden),
• false when the callback kept the window shown,
• false when called on a child Window element that can't be closed independently.

The builtin docs are updated to match (using the wording from review), and the
window_actions test now covers both the accepted and rejected paths.

Tests: env RUSTC_WRAPPER= SLINT_TEST_FILTER=window_actions cargo test --manifest-path tests/Cargo.toml -p test-driver-rust

[ogoffart]

Why don't we fix the implementation to do the right thing instead of changing the docs.
In this case, i'd say that we're more likely to want to know if the app is closed or not rather than figuring whether we are top level or not.

[tilladam]

Thanks for the review! Fixed the implementation per @ogoffart's point instead of just the docs: WindowItem::close() now returns the result of the close request (true only when the close-requested callback accepts), applied your suggested doc wording, and extended the window_actions test to cover both the accepted and rejected paths.

Also moved the changelog entry into the commit message as a changelog: line and dropped the direct CHANGELOG.md edit, per your note. Sorry for the force-push over your approval — mind re-approving when you get a chance? 🙏

[tronical]

It's already merged ;)