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.