Mac OS X: advanced options. offer to prefill system wifi PSK

Offer to prefill the wifi password of the system running
Imager, like we do on Windows.

Ask user first if that is desired, as it does cause OS X to
generate a prompt asking for admin password.

OptionsPopup.qml

@@ -438,6 +438,10 @@ Popup {
                 fieldWifiPassword.text = imageWriter.getPSK(fieldWifiSSID.text)
                 if (fieldWifiPassword.text.length) {
                     chkShowPassword.checked = false
+                    if (Qt.platform.os == "osx") {
+                        /* User indicated wifi must be prefilled */
+                        chkWifi.checked = true
+                    }
                 }
             }
         }