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/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?