Tracker: move InfoWindow to a subdirectory.

I'm going to split it into even more files, and it's easier to track
what's part of it this way.

Change-Id: I09c302da38ce96f0077c5f53fd17c08cdd6c22c9
Reviewed-on: https://review.haiku-os.org/c/haiku/+/1743
Reviewed-by: Ryan Leavengood <leavengood@gmail.com>