Fix setup-file-saver.sh
Move some type definitions from File to Buffer

The plan is to remove GtkSourceFile, and make the FileLoader and
FileSaver public.
Make the FileSaver class public

The FileSaver cannot be used as-is, there are still modifications to do.
FileSaver: don't check if the file is externally modified

This will be done only by GtkSourceBuffer, which can keep the previous
modification time and compare it with the new one.
FileSaver: do not depend on GtkSourceFile (not finished)