function watchDrags(element, grabHandler, cancelHandler, dragHandler, dropHandler) {
    return new DragWatcher(element, grabHandler, cancelHandler, dragHandler, dropHandler)
        .addListenersUntilResultInvoked();
}
@param {!HTMLElement} element
@param {!function(!Point, !MouseEvent|!TouchEvent) : void} grabHandler
@param {!function(!MouseEvent|!TouchEvent) : void} cancelHandler
@param {!function(undefined|!Point, !MouseEvent|!TouchEvent) : void} dragHandler
@param {!function(undefined|!Point, !MouseEvent|!TouchEvent) : void} dropHandler
@returns {!function() : void} Call this to dispose the watcher (removing any global callbacks it added).