mainWindow: Move some code

We will replace the Toolbar with a HeaderBar on newer versions of
Gtk, so move out some code that is shared between both paths.
mainWindow: Use a HeaderBar if available

It's the new rage :-)
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 'master' of https://github.com/fmuellner/books into fmuellner-master

Conflicts:
	mainWindow.js