Change how video dimensions are computed + update example and libraries

[LeGoffMael]

In lib:

• Currently the video dimensions are computed using ffmpeg, i think it is a heavy implementation that could be done easily using VideoPlayerController

await _video.initialize().then((_) {
  _videoWidth = _video.value.size.width;
  _videoHeight = _video.value.size.height;
});

• Add the option to change the No selection text in CoverViewer
• Update video_player library to the latest 2.2.6

In example:

• Display exported video
• Add video_player library
• Update image_picker library to the latest 0.8.4 and update code to remove deprecated implementation