Update the sample slideshow

[rbeezer]

sample-slideshow.xml could use some attention. Especially as an example of "small" PreTeXt.

• A couple of loose deprecations to address.
• At a minimum it could use managed directories and have some examples of some images.
• Much more work: perhaps it needs to become a target in the pretext/pretext script as those routines become more essential.
• And after that, into the CLI and project files.