Include A Variety Of Demo/Test Presentations

[banksJeremy]

We currently have a single example presentation and it doesn't exercise any edge cases.

Both for our own testing and for demo purposes we should include several exported presentations, exercising all features. HTML presentations, SVG presentations, mixed presentations, sub-slides, different audio formats, links and other LibreOffice features etc.

ps: Also include should-be-invalid things like overlapping slides, to make sure things don't blow up too badly.