Add keyboard navigation

[ScreamingHawk]

User has requested keyboard navigation so the app can be used on a Chromebook.

Spacebar: Start / Pause
Arrows: Forward / Back
ESC / Backspace: Go back / Exit slideshow
Enter: Select file / folder

I think this should be a simple addition, add some key listeners that trigger existing functions

[boognish-rising]

I started to create my own feature request but realized that I probably wasn't the first to think that this app would be enhanced if it supported keyboard navigation/shortcuts so upon finding this issue I figured I would just voice my support for it rather than create a duplication that would create more work & clutter and just be closed.

However I see that this is nearly two and a half years old so I'm guessing it's not an enhancement or feature that we will see anytime soon. I would offer my help, as I see there is a Help Wanted label but I'm not proficient at coding in the least. However, I would be willing to test any changes and/or debugging packages etc, if that would be of any help