Add possiblity for keyboard navigation in filesystem library view

[f0rki]

In the old stable sonata version there was ".." and "/" in the filesystem view, which allowed for quick navigation through the music library with the keyboard, which I personally was using heavily. This is missing now and I don't think there are equivalent intuitive shortcuts.
I'd like to see some possiblity to navigate through the filesystem view with the keyboard, e.g. by reintroducing ".." and "/".

[multani]

I confirm this is missing in current master.