fix: Label via accessibilityValue for some roles on macOS

[DataTriny]

See slint-ui/slint#12644

The logic comes straight from Chromium.

[DataTriny]

@John-Howard Would you please be able to check if this fixes the issue you reported? I currently don't have access to my Mac.

I tested Slint's ListView on macOS 11 a while back and VoiceOver was correctly reporting item names, the behavior may have changed in later macOS versions?

Could this by chance fix #520 at the same time?

Thanks in advance for your help.

[John-Howard]

Tested on macOS 26.6, Slint 1.17.1 (winit backend), with this branch patched in via [patch.crates-io] on accesskit / accesskit_consumer / accesskit_macos. App under test is a real 12-row Slint ListView whose rows are accessible-role: list-item + accessible-label.

The tree change works as intended. Before: row is AXStaticText, AXTitle="Synthetic project 0", AXValue missing. After: AXTitle missing, AXValue="Synthetic project 0". Window title and Role::Button toolbar controls are byte-identical across both builds.

But the rows were never silent on 26.6, so there's no symptom here for it to fix. Captured via VoiceOver's own AppleScript (content of last phrase) while arrowing through rows — stock: Synthetic task 3 text; this branch: Synthetic task 3. VoiceOver already reads the label out of AXTitle for ListBoxOption; the PR's audible effect is to drop the trailing text role description. No double-announcement in either build, listening by ear.

Our slint#12644 report was wrong about the cause, and I'm sorry for sending you after it. The rows were silent because AX focus never landed on a row — accessible-delegate-focus resolving against materialised children, which we filed separately as slint#12645. Both fixes shipped in one commit and I credited the wrong one. Isolated now by building the pre-fix commit: no row ever reports AXFocused, and VoiceOver stays on a toolbar button. Two further corrections: the version field said macOS 15, but it was macOS 26.5.1; and the reproducer as filed isn't representative — its child Text has no accessible-role: none, so it's a Role::Label node carrying the title in its own AXValue, and pasting that snippet does not produce an unreadable row.

On #520: unaffected either way. AXSelected is exposed and correct on both builds (Slint calls set_selected) — exactly one row true — yet VoiceOver announced no selection state in either. So #520 looks like an announcement issue rather than a missing attribute, and this PR neither fixes nor regresses it.

No objection to the change — it's harmless here and slightly terser — but I can't report it as fixing a real symptom on current macOS.