Desktop Mode zoom

[CrossRoast]

From what I understand, there is currently no way to zoom in and out on a document when using PdfViewer in desktop mode. It would be great to add the ability to do so, since the pdfs tend to open very zoomed out. Separate zoom buttons in the menu and/or ctrl + scroll wheel would be great to have.

[amusergrieve]

Yeah, can we get this #478 merged please? I don't want to keep using my own build that uniquely identifies myself.