+- new javascript slideshow:
+ - On show image: find this and next urls; put in place
+ those that are already here; free unneeded; initiate download of
+ the rest; if needed image is ready then initiate "transitioning", else
+ initiate "loading".
+ - On load complete: if this is the target image, initiate "transitioning".
+ - On "loading": show "loading" image
+ - On "transitioning": hide "loading" image; initiate FX animation to the
+ needed image.
+ - On animation complete: blank previous image; if "playing" then schedule
+ autoswitch to next image in the future.
+ - On autoswitch to next image: if "playing" then switch to next image.
+ - On switch to next image: if next exists, show next image, else show
+ "last image" message.
+ - On switch to prev image: if prev exists, show prev image, else show
+ "first image" message.
+ - On "play": make "playing"; switch to next image.
+ - On "stop": if "playing" cancel autoswitch; break "playing".
+ - On "start show": set up things; set "playing" state; show needed image.
+ - On "stop show": cancel any schedules, hide things.
+- make "controls" and "slideshow" two "co-routine" style classes.