Noticed this while reviewing #13529, but it's also in master @ 5ba77df. On macOS 10.13.6:

<img width="209" alt="schermafbeelding 2018-08-01 om 16 22 09" src="https://user-images.githubusercontent.com/10217/43527498-0f571db4-95a7-11e8-8d90-7863947a7689.png">


<img width="178" alt="schermafbeelding 2018-08-01 om 16 12 51" src="https://user-images.githubusercontent.com/10217/43526994-c9a515ec-95a5-11e8-96b8-9e2513e9aac5.png">

* `Show` in the action menu doesn't work if the app was hidden by `Command + H`, it does work when hidden through this menu
* send and receive don't unhide the window when opened for the action menu. It does navigate to the right tab. The other actions do work (sign, verify & debug window)

Clicking "Toon" (= macOS native show) does work, suggesting the GUI uses some non-native method of hiding.
{
    if (trayIcon)
    {
        trayIcon->setVisible(!fHideTrayIcon);
    }
}
void BitcoinGUI::showModalOverlay()
{
    if (modalOverlay && (progressBar->isVisible() || modalOverlay->isLayerVisible()))
        modalOverlay->toggleVisibility();
}
static bool ThreadSafeMessageBox(BitcoinGUI* gui, const std::string& message, const std::string& caption, unsigned int style)
{
    
    noui_ThreadSafeMessageBox(message, caption, style);
    bool modal = (style & CClientUIInterface::MODAL);
    
    
    style &= ~CClientUIInterface::SECURE;
    bool ret = false;
    
    QMetaObject::invokeMethod(gui, "message",
                               modal ? GUIUtil::blockingGUIThreadConnection() : Qt::QueuedConnection,
                               Q_ARG(QString, QString::fromStdString(caption)),
                               Q_ARG(QString, QString::fromStdString(message)),
                               Q_ARG(unsigned int, style),
                               Q_ARG(bool*, &ret));
    return ret;
}
void BitcoinGUI::subscribeToCoreSignals()