Added "Erase Metadata" option

[Zzooouhh]

Sometimes I want to open up a document file cleanly from the start, resetting the stored data for last visited page number, zoom level etc. so here is a little option to wipe those metadata clean for the target file before opening it afresh.

[vkareh]

This is a nice feature, thanks for adding it.

For context to other reviewers, since this is one of the more esoteric features in GLib: GIO stores GVFS metadata on files as you interact with them. You can see any file's metadata with gio info file.pdf | grep atril and unset any metadata attribute with gio set -d file.pdf metadata::atril::<attribute-name>. The problem is that there's no way to unset them all at once. There's interesting info about this very same issue in Evince here: https://mail.gnome.org/archives/evince-list/2018-July/msg00003.html