Home: use slideshow instead of showcase image

Things still need to be done:
- select proper images to be displayed in the slideshow
- adjust [slideshow css](https://github.com/qwinff/qwinff-site/blob/slideshow/themes/sea/source/css/jcarousel.css) and [parameters](https://github.com/qwinff/qwinff-site/blob/slideshow/themes/sea/source/js/jcarousel-init.js)