Feature request: Add support for more navigation keys

[loserMcloser]

• KP_Enter should have the same function as Return.
• Right (and KP_Right) to expand a directory, Left (and KP_Left) to collapse a directory.
• There should be a shortcut key to toggle keyboard focus between the directory tree and the preview pane so that when the keyboard focus is on the preview pane, the arrow keys and page up/down can be used to scroll in the previewed file, instead of only being able to scroll with the mouse.