AutoarExtract: refactor pathname building process

Previously, operations with pathnames obtained from libarchive were performed
directly on strings, without the use of the GFile API. This lead to an
increasing complexity of the code and also to oversights. Also, the process of
turning a path in the archive to a path on the file system had a complex logic.
It relied on multiple parameters that could have different meanings according to
some context elements, such as the number of files in an archive. One example is
the `top_level_dir` member, which could be a file if the archive had just one
element inside, or a directory in other situations.

In order to fix these issues, use the GFile API for path operations, like
finding a common prefix, path concatenation etc. For the logic part, make sure
that the top level directory is always a directory so the number of checks can
be reduced.

https://bugzilla.gnome.org/show_bug.cgi?id=768645