pdfjs: use PDFView.open instead of PDFView.load

Now we get a nice loading bar at the top of the page.

qutebrowser/browser/pdfjs.py

@@ -55,9 +55,7 @@ def _generate_pdfjs_script(url):
         url: The url of the pdf page as QUrl.
     """
     return (
-        'PDFJS.getDocument("{url}").then(function(pdf) {{\n'
-        '    PDFView.load(pdf);\n'
-        '}});'
+        'PDFView.open("{url}");\n'
     ).format(url=webelem.javascript_escape(url.toString()))