Advanced settings: use current username instead of Pi as default

Instead of using 'pi' as default, try to get the username of
the current user from the OS.

May not always work. (e.g. some Windows users may not have
setup a username themselves, but may be using a default user
created by the PC vendor).

Closes #497

src/OptionsPopup.qml

@@ -478,7 +478,10 @@ Popup {
         if ('sshUserName' in settings) {
             fieldUserName.text = settings.sshUserName
             chkSetUser.checked = true
+        } else {
+            fieldUserName.text = imageWriter.getCurrentUser()
         }
+
         if ('wifiSSID' in settings) {
             fieldWifiSSID.text = settings.wifiSSID
             if ('wifiSSIDHidden' in settings && settings.wifiSSIDHidden) {