[IMP] website_slides: adapt pdfjs 2.2.228

Use window.pdfjsLib instead of window.PDFJS.
PDF returned through getDocument() has _pdfInfo variable instead of
pdfInfo.

task-2124443

closes odoo/odoo#40917

Related: odoo/enterprise#6900
Signed-off-by: Géry Debongnie (ged) <ged@openerp.com>