Platform: add bring-all-to-front()

[tilladam]

Summary

Adds Platform.bring-all-to-front() to the Slint language, split out from
#11692 per reviewer request (orthogonal to the minimize/maximize/close work
and easier to discuss in isolation).

On macOS this calls [NSApp arrangeInFront:] — the standard implementation
of the Window › Bring All to Front menu item. On all other platforms it
is a no-op. Example usage:

MenuItem {
    title: "Bring All to Front";
    activated => { Platform.bring-all-to-front(); }
}

Design choices (addressing #11692 review feedback)

• No public Window::bring_all_to_front() in Rust, C++, or JS. The
  function lives only on the Platform global in .slint, which is the
  right home for an app-wide, window-agnostic action.
• No WindowAdapter trait method. Implemented as a free function in
  i-slint-core using objc2-app-kit directly (NSApplication feature
  was already enabled for the system tray). The C FFI entry point
  (slint_bring_all_to_front) is private_api, used only by generated
  C++ code.

Open question

@tronical raised the idea of making this cross-platform by iterating all
window adapters and calling show() on each, rather than the macOS-specific
arrangeInFront:. I've kept the macOS-only behavior for now because the
semantics are different on other platforms ("raise all our windows" is more
aggressive than the macOS concept of bringing a group of windows to the
front of the z-stack). Happy to revisit in a follow-up if there's appetite
for a cross-platform version.