Draft: ScriptFu: add dir-list to ftx extension

This is partial fix of #9074. For illustration, it won't be merged until a complete fix.

Remains to change contact-sheet.scm to use dir-list instead of the other dir- functions.

I haven't tested on Windows to determine that it actually fixes anything. But it does avoid the suspected non-portable, crashing on Windows code in the existing dir- functions.

Some of the other dir- function could also be deprecated, or just removed. They work, except seemingly on Windows. I don't know if any plugins in the wild (in user's hands) use the other dir- functions. Anyway, I think dir-list fully equivalent to the other dir- functions.

Alternatively, we could just leave the other dir- functions and just issue a warning if the platform is Windows when they are used.

The demo plugin list-home.scm also could be changed to use dir-list.

A test script should be written that fully tests all the ftx functions, including error conditions. And then execute that test script on Windows. I don't expect them to fail, since GLib should be insulating platform differences.

Since contact-sheet is not supported, no need to backport to 2.10.