Controls fix and zip-file rename

Just pushed a new build. Now you can use left/right arrow keys to switch between slides. I've also changed using middle mouse button to right mouse button to get back to previous slide.

Why it was the middle mouse button in the first place? Reason is silly. I've used Unity's default sample scene for a background for a long time (like, for actual first and very crude iteracion/idea for the app). And it happened to have this free-fly camera script attacked which was active when you've hold the right mouse button. I didn't really want to remove that option as it was kinda a fun quirk but I had to use something else to trigger previous slide. As it often happens, devs can get used to weird quirks of their apps. But now I was thinking about recording a video showcasing how to use the SimpleSlide. Saying "push the middle mouse button..." out loud just didn't feel right. So here's the update.

I've also updated the help screen as it lacked this very basic controls (no need to explain the obvious, huh?).

As for the zip-file name change. I've just used butler for the first time to upload the new buid. I didn't know it renames the file when uploading. I should have used butler from the beginning. It's really simple and fast to download and setup.