all: say Wi-Fi instead of "wifi" or "WiFi"

Correct the spelling across the *entire* tree, including translations,
comments, etc. It's easier that way.

Even the places where it's not exposed to the user, such as tests, so
that we learn how is it spelled correctly.

examples/python/dbus/show-bssids.py

@@ -18,7 +18,7 @@
 #
 
 
-# This example prints out all the AP BSSIDs that all WiFi devices on the
+# This example prints out all the AP BSSIDs that all Wi-Fi devices on the
 # machine can see.  Useful for location-based services like Skyhook that
 # can geolocate you based on the APs you can see.
 

examples/python/dbus/vpn.py

@@ -137,7 +137,7 @@ if not connection_path:
 
 device_path = get_wifi_device_path()
 if not device_path:
-    print("no wifi device found")
+    print("no Wi-Fi device found")
     sys.exit(1)
 
 # Is it already activated?