This patch completely re-implements `PDFHistory` to get rid of various bugs currently present, and to hopefully make maintenance slightly easier. Most of the interface is similar to the existing one, but it should be somewhat simplified.
The new implementation should be more robust against failure, compared to the old one. Previously, it was too easy to end up in a state which basically caused the browser history to lock-up, preventing the user from navigating back/forward. (In the new implementation, the browser history should not be updated rather than breaking if things go wrong.)
Given that the code has to deal with various edge-cases, it's still not as simple as I would have liked, but it should now be somewhat easier to deal with.
The main source of complication in the code is actually that we allow the user to change the hash of a already loaded document (we'll no longer try to navigate back-and-forth in this case, since the next commit contains a workaround).
In the new code, there's also *a lot* more comments (perhaps too many?) to attempt to explain the logic. This is something that the old implementation was serverly lacking, which is a one of the reasons why it was so difficult to maintain.
One particular thing to note is that the new code uses the `pagehide` event rather than `beforeunload`, since the latter seems to be a bad idea based on https://bugzilla.mozilla.org/show_bug.cgi?id=1336763.
133d06e9a4