Fixes #48 Select Up dir + Down + siblings

Hi Pei,

This PR replaces the previous ones #61 #62 and #63
It provides functions to quicker navigation between nodes:
- <kbd>U</kbd> to go to the up directory up to the root where pressing `U` again will change the root
- <kbd>D</kbd> somewhat the inverse of `U`, it basically selects the next expanded child or sibling
- <kbd>S</kbd> and <kbd>s</kbd> to navigate between siblings regardless if they are expanded or not

Not that the key bindings have been chosen arbitrarily and you are free to change them to anything you like.

Cheers,
syl20bnr