Modernize KJotsLinkDialog
- Get rid of custom ui-code, just use Qt Designer linkdialog.ui
- Don't use radio buttons to choose between normal URL or link to another note. Instead have just a single editable QCombobox with completer. So if user types a note or a book, we select it; otherwise we treat text as an external URL
- Becase of that, we no longer need KJotsBookshelfEntryValidator, so get rid of it.