Add preference to automatically delete videos when watched

[bcmn]

Will fix #567

I just need to wrestle with Interface Builder to add the toggle itself 🙃

[bcmn]

Closing, for all the reasons listed above 😛 We talked about this a little more elsewhere, & it's not immediately obvious how to tie it in with the existing definition of "Watched" without more work than I think we're willing to give at the moment, & when's best to safely delete the files.