file-browser: expand/collapse item on right/left keypress
When an item is selected in the file browser:
- when the left key is pressed:
- if it can be collapsed, collapse it;
- if its parent exists, go to the parent;
- otherwise, do nothing;
- when the right key is pressed:
- if it can be expanded, expand it;
- otherwise, do nothing.
This mirrors the behaviour of Nautilus
(before the tree view has been temporarily removed).