mainWindow: Set OK Button in new-book dialog as default widgets
mainWindow: Disallow new books without title or author

Rather than showing an error dialog when intenting to add a new
book without title or author, disable the confirm button until
both have been filled out.
Merge branch 'new-books' of https://github.com/fmuellner/books into fmuellner-new-books

Conflicts:
	mainWindow.js