track the currentIndex of each subdir in fs tree

[maks]

Uses a small stack to track the current position in each dir as we navigate down through a set of samples subdirs, so that we maintain prev position as we go back up the dir tree.

This makes navigating something like the AKWF subdirs a much more pleasent experience.

The stack size is 32, so only support 32 subdirs deep but that should be plenty for even extremely deeply organised sample pack file hierarchies.

Fixes: #334

[democloid]

I don't necessarily have a problem with the idea, but we are introducing a hard limit in depth, at which point we may want to warn the user in some way. How does this actually fail at that point from a user perspective? do we want to colorize directories on a different color at the last level?

I'd implement this with an actual stack before reviewing, seems cleaner.

[maks]

I don't necessarily have a problem with the idea, but we are introducing a hard limit in depth, at which point we may want to warn the user in some way. How does this actually fail at that point from a user perspective? do we want to colorize directories on a different color at the last level?

@democloid 32 levels deep is aleady a crazy deep file tree! The most I have seen in sample packs is 6 levels deep. For comparison our codebase here is max 18 deep. Also we don't have any way we show that a dir exceeds its max of file entries either and just silently truncate at the limit.