[Cyberistic]

Adds a tray / status-bar item, separate from the app menu bar.
tested on mac only
closes remorses/gpuix#109

let tray = cx.create_tray(TrayOptions {
    icon: Some("icon.png".into()),
    title: None,
    tooltip: Some("My app".into()),
})?;
tray.set_menu(vec![
    TrayMenuItem { id: "open".into(), label: "Open".into(), enabled: true, checked: false },
    TrayMenuItem { id: "quit".into(), label: "Quit".into(), enabled: true, checked: false },
]);

cx.on_tray_event(|event, cx| match event {
    TrayEvent::Click => {},
    TrayEvent::MenuItem(id) if id == "quit" => cx.quit(),
    TrayEvent::MenuItem(id) => {}
});

agent slop below:

Limits: no template-image inference, per-item icons, submenus, separators or key equivalents; the item is main-thread only (off-thread set_menu logs and is dropped); Windows needs an .ico (LoadImageW does not decode PNG), Linux decodes PNG for IconPixmap but skips .icns; duplicate ids are allowed and report the same id.