[IMP] website_slides : usability improvements

addons/website_slides/controllers/main.py

addons/website_slides/models/slides.py

addons/website_slides/static/description/index.html

addons/website_slides/static/description/slides1.png

addons/website_slides/static/description/slides2.png

addons/website_slides/static/description/slides3.png

addons/website_slides/static/description/slides4.png

...site_slides/static/lib/pdfslidesviewer/PDFSlidesViewer.js

addons/website_slides/static/src/js/slides.js

addons/website_slides/static/src/js/slides_upload.js

addons/website_slides/static/src/less/website_slides.less

addons/website_slides/static/src/xml/website_slides.xml

addons/website_slides/view/website_slides.xml

addons/website_slides/view/website_slides_backend.xml

addons/website_slides/view/website_slides_embed.xml