GFile: Add g_file_peek_path()

This is a variant of g_file_get_path() which returns a const string to
the caller, rather than transferring ownership.

I've been carrying `gs_file_get_path_cached()` in libgsystem and it
has seen a lot of use in the ostree and flatpak codebases.  There are
probably others too.

I think language bindings like Python/Gjs could also use this to avoid
an extra malloc (i.e. we could transparently replace
`g_file_get_path()` with `g_file_peek_path()`.

(Originally by Colin Walters. Tweaked by Philip Withnall to update to
2.56, change the function name and drop the locking.)

https://bugzilla.gnome.org/show_bug.cgi?id=767976

docs/reference/gio/gio-sections.txt

@@ -91,6 +91,7 @@ g_file_hash
 g_file_equal
 g_file_get_basename
 g_file_get_path
+g_file_peek_path
 g_file_get_uri
 g_file_get_parse_name
 g_file_get_parent