Better YouTube EN

Currently-implemented features are below, but I will add more features later.

1. Easy audio gain change.
This feature is inspired by the "SoundFixer" extension. That extension was helpful, but it was not so convenient for me to use it, because it was not specifically designed for YouTube. Better YouTube adds quick gain buttons right below the volume slider, so that one can easily change the gain and the volume in conjunction, which was very necessary from my personal experience.

Also, when opening a new video, the gain is reset to 1. This is needed, because otherwise, if I set a high audio gain on a low-volume-level video and then open a new video with a higher volume level, it will be very loud.

One known issue I have noticed is that when audio gain is set, there is a delay (about 1 second) when changing the volume with the YouTube's volume slider. But this was also observed when using "SoundFixer", so probably this is an inevitable result of using AudioContext.

2. Video thumbnail viewer.
I often need to see the video thumbnail again whilst watching the video to determine if the thumbnail was a click-bait. Clicking the "Thumb" button opens the thumbnail of the best resolution in a new window.

3. Profile picture viewer.
No more wondering "What the hell is that picture" when seeing a blurry small profile picture in the comment section. Clicking the profile thumbnail shows the full-size profile picture at the centre of the page, instead of going to the commenter's channel.

4. Toggle video player UI (e.g., seek bar, play/pause buttons, etc).
This is needed when I tried to look the bottom of a video carefully (something like hard-coded subtitle) pausing the video, the video control UI blocked my view.

5. Skip (fast-forward) video for tens of seconds.
You could do it without this plug-in by pressing arrow keys or dragging the knob on the seek bar, but this is more convenient, because it is just one mouse click.

6. Toggle YouTube interaction tools (e.g., Like, Dislike, Share, etc).
This is needed because if the screen width is not enough, it blocks the publication date or the view count.