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.
#ifndef BITCOIN_QT_MACDOCKICONHANDLER_H
#define BITCOIN_QT_MACDOCKICONHANDLER_H
#include <QObject>
/** macOS-specific Dock icon handler.
 */
class MacDockIconHandler : public QObject
{
    Q_OBJECT
public:
    static MacDockIconHandler *instance();
    static void cleanup();
Q_SIGNALS:
    void dockIconClicked();
private:
    MacDockIconHandler();
};
#endif