Make sure that keyboard shortcuts match common apps/remotes

[mirisuzanne]

What keypress events do other apps expect, and remotes send for…

• previous/next (I think these are covered)
• start/stop presentation
• fullscreen?
• blank slide
• etc…

[mirisuzanne]

@stacyk and @jerivas both raised issues and questions around what shortcuts we have, and how they work across different devices. Figured we should re-open this issue, and define exactly what keyboard-shortcuts we want.

[jerivas]

I created a doc so we can add the shortcuts we want and discuss them with the Drive comment system. It has worked well in the past for things like this.

I tried to add everything from the README and provided Windows alternatives for some common actions. Feel free to edit if I mistyped something, especially on the Mac side.