Implement a thumbnailer for Linux

[ISSOtm]

The new XdgThumbnailer implements https://wiki.gnome.org/Attic/DraftThumbnailerSpec#Specialized_thumbnailers, using Gio as its D-Bus back-end.

Not anymore since 4cc34f97ecc37c8960b06372ec4a1a4153a79c56. The PR now implements this other GNOME "spec" instead, which Tumbler (XFCE's thumbnailer) also implements (and reportedly KDE as well), and that everything seems to be using (e.g. Aseprite). The nice bonus is that this requires far less code, since really all of the "plumbing" is handled by the thumbnailer daemon.

[ISSOtm]

Respects paid to @max-m for his own implementation giving me inspiration to dive into this PR.

I tested this locally, and this PR is now definitely ready for review!