* [pdf viewer] added loading icon; adjusted the position of the scrollbar of the viewer container
* [pdf file view] fixed UI of the image in the details panel
* [pdf viewer] use the latest PDFjs(v3.8.162)
* [pdf viewer] redesigned it: added side panel(thumbnail panel) and bottom bar(includes side panel toggle, 'file page manager', and 'zoom manager')
* [pdf file view] added 'print' to the top-right toolbar