filesystemmodel: Retire GtkFileSystemItem

filesystemmodel: Retire GtkFileSystemItem

This has to be the shortest-living object in GTK history!

It helped us greatly during the transition to GtkColumnView, but
now we can remove it in favour of GFileInfo directly. Perhaps I
could have never introduced GtkFileSystemItem in the first place,
but we're 30 commits deep and it's too late to just redo the whole
thing that will get us exactly here anyway.