[Prototype] PDF browsing history

This PR implements a browsing history prototype for PDF files.
The back/forward functionality is currently accessed either from the context menu, or even simpler by using `Alt + left arrow`/`Alt + right arrow`.
With this PR it is possible to click on a link within a PDF file, and then return to the previous page.

Related issues: #900, #750 and [bug 842670](https://bugzilla.mozilla.org/show_bug.cgi?id=842670)