Xreader option to reopen all open documents on restart

[davidfetter]

In my use case, and I suspect in many others', I'll be looking into something and have several documents open in Xreader, and then shut it down for whatever reason: kernel upgrades, for example. My window manager dutifully reopens all the Xreader windows, but they are all opened empty, and I then need to figure out which of the "recents" were in fact open.

I'd like to propose an option, defaulted to off because on could have the effect of consuming resources people can't spare on startup, that reopens all documents that were open when it got shut off.

I'm happy to give patching this a try if that's needed.