Implement frame-to-frame video navigation

[ToukL]

• Uses a hack to navigate frame-to-frame in video, based on requestVideoFrameCallback and https://github.com/angrycoding/requestVideoFrameCallback-prev-next

WARNING: this works only in Chrome (not in Firefox). Not tested in other navigators.

• Adds a 'Jump step' setting to directly jump a certain amount of seconds backward and forward

Changes video navigation buttons:

• step-backward and step-forward are used to go to previous and next frames in video
• add double-backward and double-forward buttons for Jump step setting
• add chevron-left and chevron-right to go to previous/next video of volume

[mzur]

This is an interesting approach! I need to test this thoroughly with different videos because I had so much trouble with this in the past. There are also a couple of things that could be streamlined for the final implementation but I'll get to that once I tested it a bit.

However, this also implements the "jump forward/backward by a fixed number of seconds", right (#262)? Could you please split this from the "jump frame-by-frame" changes and send both in separate PRs? The jumping by seconds can be merged much sooner then.

[mzur]

For the record: I mentioned the workaround in the W3C discussion I follow and others say that it might not produce perfect results in all cases. This article recommends the WebCodecs API for frame-accurate applications. I had a look at this, too, but as the linked StackOverflow answer mentions, it only works if the media is demuxed. And I think there is no easy solution for that if we still allow many different video file formats in BIIGLE.

[ToukL]

I did split the PR in two.

I know it's just a workaround that needs more testing and cleaning, if you have time and ideas for that it would be great ! I implemented it like that because we need the frame-by-frame navigation, even if not perfect fro now. I'm open to any suggestion though !

[mzur]

Thanks! We'll continue in the new PRs.