Improve usability of CSR creation

* Do not set an empty card holder name; this would clear the preset value
* Create input widgets before DN widget; this makes fixTabOrder()
  superfluous and automatically gives input focus to the first
  input widget
* Select the preset text in the first line edit; this allows the user
  to overwrite it easily
* Explicitly call the slots connected to textChanged() signals once
  the setup is complete
* Remember the attribute values entered by the user as presets for
  the next CSR creation

GnuPG-bug-id: 5127