CommonLibrary had some undesired knowledge of UI. Split AppPreference
and UIPreference. Then move some more stuff from AppUI* to UILibrary.
WARNING: this forgets existing UI preferences (e.g. favorite servers).
- Let the user close the window, the app will just remain alive in the
status bar
- Accordingly, replace "Confirm quit" preference with the option to stay
alive in the status bar
- Add "About..." item