homescreen 2017: update wifi status icon using network binding

The WiFi, BT, and WWAN status bar indicators are currently updated using an exposed DBus API. In order to completely deprecate the old WiFi binding, it is necessary to update at least the WiFi indicator via the networking binding events. Since the Bluetooth binding hasn't yet been rewritten, we should continue to allow that to be updated via DBus, and just toggle the WiFi icon from network binding events.

Note: once the BT indicator is also driven from appfw events, the StatusBarServer DBus api will be deprecated and can be removed.