Help test a snap version

["Michael Terry" <mike mterry name>]

Hello! For a variety of reasons [1], I've been looking at supporting a snap and/or flatpak version of deja-dup.

A flatpak version isn't ready yet [2]. But the snap version of deja-dup is feature complete (only missing nautilus integration [3], to my knowledge). And I've got a version for testing that I'd like folks to bang on a little bit.

Test instructions:

- Make sure snap is set up correctly

- Optionally uninstall deja-dup (I'm curious to hear reports of problems with either fresh snap installs or parallel installs with a system deja-dup)

- Run "snap install deja-dup --classic --candidate"

- Open deja-dup with "snap run deja-dup" or finding it in the application list (the snap uses our new icon) or just "deja-dup" if you don't have a system version already installed.

- You might want to also log out / log in or kill the current deja-dup-monitor to let the new monitor start.

(One gotcha is that snap's autostart support requires the app to be run first before we can install the autostart file. So that's a known issue that would be more difficult to workaround than I think it's worth.)

 I've marked it as 38.4 - I probably should push out a real 38.4 tarball before making this public.

Thanks in advance to anyone that gives it a go!

[1] Here a a few:

- Easier to get fixes for deja-dup and duplicity to users

- Can guarantee the dependencies we need for our various backends are installed

- Which is good, since packagekit (which we use to install missing dependencies) seems on the decline - see https://blogs.gnome.org/hughsie/2019/02/14/packagekit-is-dead-long-live-well-something-else/

- We can include build secrets, which are necessary to support some cloud backends (like if we wanted to use our own Google API key for Google Drive, we can't just commit that to a public repo)

[2] Flatpak is missing some features like autostarting, that we'd want to use. So I haven't prioritized it. But those features are being worked on, so eventually I hope to support an install on flathub or similar.

[3] Neither flatpak nor snap are likely to let us install an integrated nautilus extension. We should think longer term about building our own UI for restoring single or missing files. It's a common complaint that folks don't even think to look in nautilus for that feature anyway.