Sometimes, while working with the files_view_get_selection function, the
freeing is not done properly. We leak a lot of objects as of this and we
need a way to fix it.

To avoid leaking, we implemented g_autoptr support for NautilusFile in order
to have at our disposal automatic, deep cleanup whenever we use the function.

In this patch, we changed all calls to use g_autolist appropriately and bumped
the glib dependency up to 2.55.1 (in order for us to use g_autolist).

Fixes GNOME/nautilus#226