Add plt-title-background to slideshow

[dfeltey]

This pull request adds the plt-title-background slide to slideshow to avoid dependencies on unstable in slideshow presentations using the pict.

[rfindler]

@mflatt what do you think about this, as opposed to a separate package? After a little discussion here, we feel like there are a few options.

1. cast a wider net and try to make some sense out of all of the pict-related pkgs on the pkg server, possibly coming up with some new names (but maybe that's too much work for now)

2. make this be a separate pkg, possibly combined with @samth 's pkg that has the langs-in-racket slide (but there are discoverability issues)

3. merge this here and look into those other options another day.

[mflatt]

Sorry that I didn't see this pull request (or the mention) until now... It feels to me like something that belongs in its own package. I don't think it needs to be combined with anything else; I think small packages are fine.