SlideDirection -> SlideTransition

[yonomitt]

In the future, it might be handy to add more transitions between slides. For instance, in Keynote, I use Dissolve quite often, and would miss it here. This would require adding a new case to the SlideDirection enum.

However, at the point of adding more transition types, the name no longer makes sense. It might be better to rename it to SlideTransition to more accurately reflect the functionality.

[joshdholtz]

I completely agree with this! I will get this renamed 😊