traverse by directory modified date instead of by name

Sorting alphabetically only works nicely if changes are made to the
lexically latest folder, but not if changes happen in an old folder.

tools/detection/detector.py

@@ -206,7 +206,7 @@ def recurse(folder, queue, bar):
 
         # walk newer folders first to hide slow remote file system enumeration
         # while already processing the first videos
-        subdirs.sort(reverse=True)
+        subdirs.sort(key=lambda sdir: os.path.getmtime(os.path.join(parent, sdir)), reverse=True)
 
         for name in files:
             (_, ext) = os.path.splitext(name)