applet, connection-editor: Use "Ethernet" and "Wi-Fi" in the UI

Change "Wired" to "Ethernet" and "Wireless" to "Wi-Fi" everywhere in
the UI, to more clearly reflect what they mean. (ie, "Wired" did not
include DSL or InfiniBand, and "Wireless" did not include mobile
broadband or WiMAX.)

Also, fix some places that used "wifi"/"Wifi"/"WiFi" instead of
"Wi-Fi".