Out sourced validators to a separate class.
Renamed the URL field to address and replaced the text field by a label.
Added mechanism that changes in the textfields are only stored, if they differ from the value before. Otherwise, the keys were always encrypted again, what took of course some time and led to a short delay (because of PBKDF2).
Fixed a bug where the panel didn't react when the server was null.
Added a default value to the server to avoid NullPointers.
Added a QR code to the window. It's only visible, when the server is configured.