Fetch recommended-downloads from a remote server

This allows for distributors to host their very own recommended-downloads.xml file and point Boxes installations to it. This is a solution to the problem of updating the recommended-downloads in previous/old gnome-boxes versions.

One can set the org.gnome.boxes recommended-downloads-url gsetting key to override the default value.

If the key is set to empty (""), Boxes will use the recommended-downloads.xml file in cache, and if there's no cache, it fallbacks to the recommended-downloads.xml file shipped/bundled in $SOURE/data/recommended-downloads.xml

The default URL is https://gnome.pages.gitlab.gnome.org/gnome-boxes-logos/recommended-downloads.xml which is a file hosted in the same repository that hosts the distro logos used by Boxes.