window: make Alt-Down behavior symmetric to Alt-Up

(This is my take on !961 (merged))

Previously Alt-Down was an alternative to Enter. It opened folders and files. All other Alt-[arrow key] combinations only navigate, so opening of files was strange behavior for a navigation shortcut. Similarly, Alt-Down and Alt-Up did not form a symmetric pair, like Alt-Left and Alt-Right.

So to create the symmetry, change Alt-Down to navigate into previously opened folders, if those folders reside in the current location. This allows for a new way of easily navigating up to a parent folder with Alt-Up and quickly navigating back to the previous location with Alt-Down, without risking to overshoot beyond the desired location, as e.g. navigation with Alt-Left can cause.