Video caching improvements

[KevMCarp]

Currently the videos are downloaded and cached before attempting to play.

In an ideal world, we would download, cache and play simultaneously. However the media_kit package doesn't support this workflow.

One way to improve the user experience would be to download and cache whilst media_kit also streams the video.
On subsequent plays, the video would then be retrieved from cache.

The downside of this method is that all videos would be downloaded twice on the first play