Included PDF.js and possiblity to show pdfs

This merge would fix #900.

Currently you are able to show PDFs on the projector. To do this, you have a widget where you can "activate" a pdf, choose the page and go to the next / previous page. I also want to implement a funtion to show all this in fullscreen and that you dont have to reload the pdf on each change of the page. But to do this, i have to wait until there is a way to send commands via a websocket to the client.