fix CApplicationMessenger::SendText() not sending the text to the proper edit control

@Tolriq told me that JSON-RPC's `Input.SendText` doesn't work in some dialogs so I tried to reproduce it and realized that it only worked by chance. I always assumed that sending a `CGUIMessage` with a control ID of `0` would simply go to the currently focused control but apparently that's not the case. So we have to get the ID of the currently focused control ourselves.