Wifi menu status

New feature to add an icon to the menu to show Wifi status. The Wifi icon shows as greyed out if Wifi is not available. You can touch the icon to prompt to switch Wifi on/off as appropriate.

Small change to NetworkMgr:getWifiStatus() to ping gateway rather than 8.8.8.8, as wifi may be connected with no Internet access!
Testing with `NetworkMgr:getWifiStatus` is better than testing the appearance of the icon which is not (always) necessarily the wifi status.