Support for exploded views of deb822 entries

Provide an exploded_list view of the list where each entry represents a single release file (1:n relation, vs n:m), matching legacy sources.list entries.

On save, merge the exploded entries back together as far as possible. This is currently lacking perhaps as we do not reorder the entries to accommodate a change in the middle.

Change aptsources.distro to use the exploded view.

Change the remove() code to be able to remove exploded entries from the SourcesList.

This makes software-properties recognize Ubuntu Sources correctly when they are stored in multi-value deb822 entries, but modifying them does not work so far, presumably due to the separation of the privileged daemon from the graphical application.

Test cases also need to be written.