Add support for client-side PDF export

Develop some Javascript to bridge XWiki and the Browser's print feature, when clicking on the Export to PDF button in XWiki's UI:

• Add a loading animation
• Compute the print preview URL based on the PDF options (XWIKI-19267)
• Inject a hidden iframe into the current page pointing it to the print URL
• Wait for the iframe to load and also for the JavaScript within the iframe to be ready (XWIKI-19266)
• Remove the loading animation
• Call iframe.contentWindow.print()
• The user is free to either download the PDF or send it to a printer
• Also adapt CSS for printing so that links are displayed more nicely than the default (by default Chrome displays the link text + the link URL next to it)