* imglab: select next/previous image with 's' and 'w'

* imglab: make 'w' and 's' keys behave like UP and DOWN keys; add 'about' text

because it's a much less stressing hand motion to hit END that i in most cases.

next using dlib::correlation_tracker.

histogram is equalized or unmodified.  This way, if you are looking at
particularly dark or badly contrasted images you can toggle this mode and maybe
get a better view of what you are labeling.

Changed from ctrl to alt because otherwise it's easy to accidentally hit ctrl up and jump when you don't meant to while pruning an image list.

small window when the first image was bigger than the screen.

auto label boxes.  It will also auto-skip empty images when you hold ctrl.

to change the label for a rectangle.

to/from the xml files.

jump to the beginning.

first image.