gnome-desktop is not supposed to be used by applications, that's why
it's not in the runtime. And we have an Epiphany-specific security
problem in the thumbnailer, which we cannot otherwise fix without adding
new gnome-desktop API that would only be useful for Epiphany.

The thumbnail factory is a lot of code, and we only need a small bit of
it.

In particular, get rid of the thumbnail mtime tracking code, which is
unnecessary now that we control the thumbnailing code. For the past
couple of years, it has only been used to convince the thumbnail factory
to not discard our requests for thumbnails.

Also, save the snapshots under ~/.cache/epiphany, so that we can delete
them. Right now, we store undeletable snapshots of all the websites you
view, which don't go away when clearing history (except, fortunately, in
incognito mode). Now we can fix that.

This commit also contains a minor memory leak fix, for good measure.

https://bugzilla.gnome.org/show_bug.cgi?id=778578