Feature request: Show progress bar for loading videos/media

[mcclure]

Something to consider once #3857 is merged:

One of my big frustrations with Tusky is I will often tap a video (or sometimes even a large image) and I find myself just staring at that ⟳ icon. At this point I don't know: (1) It's loading, just very slowly (2) My cellphone service has dropped and it's frozen (3) mastodon.social has gone responsive and it's frozen [this actually happens pretty frequently] (4) Tusky has hit some sort of failure loading the video.

I would love a progress bar for slow-loading media so I can meaningfully answer the question "is it going to load or should I give up?". Maybe it could wait some number of seconds (5?) to display and have enough of a simple text label to be able to tell the difference between "Server has not sent a response…" and "Server has sent response headers but 0% of the binary has been sent…".