Re-named menu command so it's clearer (and adjusted doc).

Crop follows the *median* of instance centroids (actually it's a
moving average to smooth crop movement).

In the GUI you can select none/half/quarter crops; the cli allows
you to specify exact x,y crop size. There's currently no checking
that crop size is <= image size.

For issue #233.

For issue #291.

Previously there would be an error when trying to stack frames
if we didn't get any.

Don't require labels filename, check for skeleton in config and
in val labels (and don't die if not found).

Added human-readable labels for metrics in detailed dialog.

Increased subdir search depth.