Code review comment for lp:~j-corwin/openlp/present

Looks good to me too.

Just some info: in v1.x when a presentation is "loaded" initially (via the media manager),
that is when the previews are generated. I don't know how we'd be able to detect when a
slide or a presentation has changed, but perhaps we should copy the presentation into a
"cache" directory (the Linux XDG system provides a cache dir, we can create one on Mac OS X
and Windows) and then, as per v1.x, use a "refresh" system if a person wants to reload
their presentation after it has changed.