The default had been .tar.xz, but as claimed in [1], it makes back up
significantly slower with not much gain on the back up file size. This
change adds a setting to remember what file type had been used the last
time and it is reused the next time the back up is invoked.
[1] https://bugzilla.redhat.com/show_bug.cgi?id=1491024
The API of gnome-autoar was recently modified so compression and extraction
were no longer working. Replace autoar preferences objects with two shell
settings. Save memory file to disk before extracting it. Handle generation of
unique file names internally.
https://bugzilla.gnome.org/show_bug.cgi?id=770380
