Qt: Add option to hide the system tray icon

My changes leave all tray icon and menu creation/initialization logic
untouched. It only shows or hides the icon according to the setting.

A new checkbox was added to the OptionsDialog under the Window tab. A
bool option named "hideTrayIcon" was added to OptionsModel. This
checkbox was mapped like other all options to the OptionsModel.

A signal was added to the OptionsModel for broadcasting changes the the
hideTrayIcon option. This signal was connected to a new slot added to
BitcoinGUI named setTrayIconVisible(bool). The slot simply hides or
shows the trayIcon in BitcoinGUI according to the parameter recieved.

src/qt/optionsmodel.h

@@ -28,6 +28,7 @@ public:
 
     enum OptionID {
         StartAtStartup,         // bool
+        HideTrayIcon,           // bool
         MinimizeToTray,         // bool
         MapPortUPnP,            // bool
         MinimizeOnClose,        // bool
@@ -58,6 +59,7 @@ public:
     void setDisplayUnit(const QVariant &value);
 
     /* Explicit getters */
+    bool getHideTrayIcon() { return fHideTrayIcon; }
     bool getMinimizeToTray() { return fMinimizeToTray; }
     bool getMinimizeOnClose() { return fMinimizeOnClose; }
     int getDisplayUnit() { return nDisplayUnit; }
@@ -72,6 +74,7 @@ public:
 
 private:
     /* Qt-only settings */
+    bool fHideTrayIcon;
     bool fMinimizeToTray;
     bool fMinimizeOnClose;
     QString language;
@@ -87,6 +90,7 @@ private:
 Q_SIGNALS:
     void displayUnitChanged(int unit);
     void coinControlFeaturesChanged(bool);
+    void hideTrayIconChanged(bool);
 };
 
 #endif // BITCOIN_QT_OPTIONSMODEL_H